| Message ID | 699121e5c938c6f4b7b14a7e2648fa15af590a4a.1559623368.git.viresh.kumar@linaro.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: arm64: Drop 'const' from argument of vq_present() | expand |
On 04/06/2019 05:43, Viresh Kumar wrote: > We currently get following compilation warning: > > arch/arm64/kvm/guest.c: In function 'set_sve_vls': > arch/arm64/kvm/guest.c:262:18: warning: passing argument 1 of 'vq_present' from incompatible pointer type > arch/arm64/kvm/guest.c:212:13: note: expected 'const u64 (* const)[8]' but argument is of type 'u64 (*)[8]' Using which toolchain? My GCC 8.3.0 doesn't say anything. Thanks, M.
On 04-06-19, 09:30, Marc Zyngier wrote: > On 04/06/2019 05:43, Viresh Kumar wrote: > > We currently get following compilation warning: > > > > arch/arm64/kvm/guest.c: In function 'set_sve_vls': > > arch/arm64/kvm/guest.c:262:18: warning: passing argument 1 of 'vq_present' from incompatible pointer type > > arch/arm64/kvm/guest.c:212:13: note: expected 'const u64 (* const)[8]' but argument is of type 'u64 (*)[8]' > > Using which toolchain? My GCC 8.3.0 doesn't say anything. I haven't updated mine since a long time it seems :) aarch64-linux-gnu-gcc (Linaro GCC 4.9-2015.05) 4.9.3 20150413 (prerelease)
On Tue, Jun 04, 2019 at 10:13:19AM +0530, Viresh Kumar wrote: > We currently get following compilation warning: > > arch/arm64/kvm/guest.c: In function 'set_sve_vls': > arch/arm64/kvm/guest.c:262:18: warning: passing argument 1 of 'vq_present' from incompatible pointer type > arch/arm64/kvm/guest.c:212:13: note: expected 'const u64 (* const)[8]' but argument is of type 'u64 (*)[8]' Since the vq_present() function does not modify the vqs array, I don't understand why this warning. Compiler bug?
On 04-06-19, 09:43, Catalin Marinas wrote: > On Tue, Jun 04, 2019 at 10:13:19AM +0530, Viresh Kumar wrote: > > We currently get following compilation warning: > > > > arch/arm64/kvm/guest.c: In function 'set_sve_vls': > > arch/arm64/kvm/guest.c:262:18: warning: passing argument 1 of 'vq_present' from incompatible pointer type > > arch/arm64/kvm/guest.c:212:13: note: expected 'const u64 (* const)[8]' but argument is of type 'u64 (*)[8]' > > Since the vq_present() function does not modify the vqs array, I don't > understand why this warning. Compiler bug? Probably yes. Also marking array argument to functions as const is a right thing to do, to declare that the function wouldn't change the array values. I tried a recent toolchain and this doesn't happen anymore. Sorry for the noise.
On 04/06/2019 09:43, Catalin Marinas wrote: > On Tue, Jun 04, 2019 at 10:13:19AM +0530, Viresh Kumar wrote: >> We currently get following compilation warning: >> >> arch/arm64/kvm/guest.c: In function 'set_sve_vls': >> arch/arm64/kvm/guest.c:262:18: warning: passing argument 1 of 'vq_present' from incompatible pointer type >> arch/arm64/kvm/guest.c:212:13: note: expected 'const u64 (* const)[8]' but argument is of type 'u64 (*)[8]' > > Since the vq_present() function does not modify the vqs array, I don't > understand why this warning. Compiler bug? Yeah, I'm a bit puzzled by that one. Also checked with clang, which is usually much more picky about things, but it didn't complain. We could drop the const without much harm, but I really wish we weed out these ancient compilers... Thanks, M.
On Tue, Jun 04, 2019 at 02:25:45PM +0530, Viresh Kumar wrote: > On 04-06-19, 09:43, Catalin Marinas wrote: > > On Tue, Jun 04, 2019 at 10:13:19AM +0530, Viresh Kumar wrote: > > > We currently get following compilation warning: > > > > > > arch/arm64/kvm/guest.c: In function 'set_sve_vls': > > > arch/arm64/kvm/guest.c:262:18: warning: passing argument 1 of 'vq_present' from incompatible pointer type > > > arch/arm64/kvm/guest.c:212:13: note: expected 'const u64 (* const)[8]' but argument is of type 'u64 (*)[8]' > > > > Since the vq_present() function does not modify the vqs array, I don't > > understand why this warning. Compiler bug? > > Probably yes. Also marking array argument to functions as const is a > right thing to do, to declare that the function wouldn't change the > array values. > > I tried a recent toolchain and this doesn't happen anymore. > > Sorry for the noise. Sparse is already warning about this, but I had dismissed it as a false positive. I think this is an instance of disallowing implicit conversions of the form T ** -> T const ** because this allows a const pointer to be silently de-consted, e.g.: static const T bar; void foo(T const **p) { *p = &bar; } T *baz(void) { T *q; foo(&q); return q; } I _suspect_ that what's going on here is that the compiler is eliminating a level of indirection during inlining (i.e. converting pass-by-reference to direct access, which is precisely what I wanted to happen). This removes the potentially invalid behaviour as a side-effect. This relies on the compiler optimising / analysing the code aggressively enough though. So, I don't have a problem with dropping the extra extra const, e.g.: static bool vq_present( u64 (*const vqs)[KVM_ARM64_SVE_VLS_WORDS], unsigned int vq) Since this function is static and only used very locally, I don't see a big risk: the only reason for the extra const was to check that vq_present() doesn't modify vqs when it shouldn't. But it's a trivial function, and the intent is pretty clear without the extra type modifier. I'm in two minds about whether this is worth fixing, but if you want to post a patch to remove the extra const (or convert vq_present() to a macro), I'll take a look at it. Cheers ---Dave
On 04-06-19, 10:26, Dave Martin wrote: > I'm in two minds about whether this is worth fixing, but if you want to > post a patch to remove the extra const (or convert vq_present() to a > macro), I'll take a look at it. This patch already does what you are asking for (remove the extra const), isn't it ? I looked at my textbook (The C programming Language, By Brian W. Kernighan and Dennis M. Ritchie.) and it says: " The const declaration can also be used with array arguments, to indicate that the function does not change that array: int strlen(const char[]); " and so this patch isn't necessary for sure.
On Tue, Jun 04, 2019 at 03:01:53PM +0530, Viresh Kumar wrote: > On 04-06-19, 10:26, Dave Martin wrote: > > I'm in two minds about whether this is worth fixing, but if you want to > > post a patch to remove the extra const (or convert vq_present() to a > > macro), I'll take a look at it. > > This patch already does what you are asking for (remove the extra > const), isn't it ? Yes, sorry -- I didn't scroll back far enough. > I looked at my textbook (The C programming Language, By Brian W. > Kernighan and Dennis M. Ritchie.) and it says: > > " > The const declaration can also be used with array arguments, to > indicate that the function does not change that array: > > int strlen(const char[]); > " > > and so this patch isn't necessary for sure. This is an array to which a pointer argument points, not an array argument. I think that's how we hit the constified double-indirection problem. Cheers ---Dave
On Tue, Jun 04, 2019 at 10:13:19AM +0530, Viresh Kumar wrote: > We currently get following compilation warning: > > arch/arm64/kvm/guest.c: In function 'set_sve_vls': > arch/arm64/kvm/guest.c:262:18: warning: passing argument 1 of 'vq_present' from incompatible pointer type > arch/arm64/kvm/guest.c:212:13: note: expected 'const u64 (* const)[8]' but argument is of type 'u64 (*)[8]' > > The argument can't be const, as it is copied at runtime using > copy_from_user(). Drop const from the prototype of vq_present(). > > Fixes: 9033bba4b535 ("KVM: arm64/sve: Add pseudo-register for the guest's vector lengths") > Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org> > --- > arch/arm64/kvm/guest.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c > index 3ae2f82fca46..78f5a4f45e0a 100644 > --- a/arch/arm64/kvm/guest.c > +++ b/arch/arm64/kvm/guest.c > @@ -209,7 +209,7 @@ static int set_core_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg) > #define vq_mask(vq) ((u64)1 << ((vq) - SVE_VQ_MIN) % 64) > > static bool vq_present( > - const u64 (*const vqs)[KVM_ARM64_SVE_VLS_WORDS], > + u64 (*const vqs)[KVM_ARM64_SVE_VLS_WORDS], > unsigned int vq) > { > return (*vqs)[vq_word(vq)] & vq_mask(vq); Ack, but maybe this should just be converted to a macro? It already feels a bit like overkill for this to be a function. Cheers ---Dave
On 04-06-19, 10:59, Dave Martin wrote: > On Tue, Jun 04, 2019 at 10:13:19AM +0530, Viresh Kumar wrote: > > We currently get following compilation warning: > > > > arch/arm64/kvm/guest.c: In function 'set_sve_vls': > > arch/arm64/kvm/guest.c:262:18: warning: passing argument 1 of 'vq_present' from incompatible pointer type > > arch/arm64/kvm/guest.c:212:13: note: expected 'const u64 (* const)[8]' but argument is of type 'u64 (*)[8]' > > > > The argument can't be const, as it is copied at runtime using > > copy_from_user(). Drop const from the prototype of vq_present(). > > > > Fixes: 9033bba4b535 ("KVM: arm64/sve: Add pseudo-register for the guest's vector lengths") > > Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org> > > --- > > arch/arm64/kvm/guest.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c > > index 3ae2f82fca46..78f5a4f45e0a 100644 > > --- a/arch/arm64/kvm/guest.c > > +++ b/arch/arm64/kvm/guest.c > > @@ -209,7 +209,7 @@ static int set_core_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg) > > #define vq_mask(vq) ((u64)1 << ((vq) - SVE_VQ_MIN) % 64) > > > > static bool vq_present( > > - const u64 (*const vqs)[KVM_ARM64_SVE_VLS_WORDS], > > + u64 (*const vqs)[KVM_ARM64_SVE_VLS_WORDS], > > unsigned int vq) > > { > > return (*vqs)[vq_word(vq)] & vq_mask(vq); > > Ack, but maybe this should just be converted to a macro? I will send a patch with that if that's what you want. Thanks.
On Fri, Jun 07, 2019 at 11:30:37AM +0530, Viresh Kumar wrote: > On 04-06-19, 10:59, Dave Martin wrote: > > On Tue, Jun 04, 2019 at 10:13:19AM +0530, Viresh Kumar wrote: > > > We currently get following compilation warning: > > > > > > arch/arm64/kvm/guest.c: In function 'set_sve_vls': > > > arch/arm64/kvm/guest.c:262:18: warning: passing argument 1 of 'vq_present' from incompatible pointer type > > > arch/arm64/kvm/guest.c:212:13: note: expected 'const u64 (* const)[8]' but argument is of type 'u64 (*)[8]' > > > > > > The argument can't be const, as it is copied at runtime using > > > copy_from_user(). Drop const from the prototype of vq_present(). > > > > > > Fixes: 9033bba4b535 ("KVM: arm64/sve: Add pseudo-register for the guest's vector lengths") > > > Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org> > > > --- > > > arch/arm64/kvm/guest.c | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c > > > index 3ae2f82fca46..78f5a4f45e0a 100644 > > > --- a/arch/arm64/kvm/guest.c > > > +++ b/arch/arm64/kvm/guest.c > > > @@ -209,7 +209,7 @@ static int set_core_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg) > > > #define vq_mask(vq) ((u64)1 << ((vq) - SVE_VQ_MIN) % 64) > > > > > > static bool vq_present( > > > - const u64 (*const vqs)[KVM_ARM64_SVE_VLS_WORDS], > > > + u64 (*const vqs)[KVM_ARM64_SVE_VLS_WORDS], > > > unsigned int vq) > > > { > > > return (*vqs)[vq_word(vq)] & vq_mask(vq); > > > > Ack, but maybe this should just be converted to a macro? > > I will send a patch with that if that's what you want. I think this would solve the problem and simplify the code a bit at the same time. So go for it. Cheers ---Dave
diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c index 3ae2f82fca46..78f5a4f45e0a 100644 --- a/arch/arm64/kvm/guest.c +++ b/arch/arm64/kvm/guest.c @@ -209,7 +209,7 @@ static int set_core_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg) #define vq_mask(vq) ((u64)1 << ((vq) - SVE_VQ_MIN) % 64) static bool vq_present( - const u64 (*const vqs)[KVM_ARM64_SVE_VLS_WORDS], + u64 (*const vqs)[KVM_ARM64_SVE_VLS_WORDS], unsigned int vq) { return (*vqs)[vq_word(vq)] & vq_mask(vq);
We currently get following compilation warning: arch/arm64/kvm/guest.c: In function 'set_sve_vls': arch/arm64/kvm/guest.c:262:18: warning: passing argument 1 of 'vq_present' from incompatible pointer type arch/arm64/kvm/guest.c:212:13: note: expected 'const u64 (* const)[8]' but argument is of type 'u64 (*)[8]' The argument can't be const, as it is copied at runtime using copy_from_user(). Drop const from the prototype of vq_present(). Fixes: 9033bba4b535 ("KVM: arm64/sve: Add pseudo-register for the guest's vector lengths") Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org> --- arch/arm64/kvm/guest.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)