diff mbox series

[Part2,v5,34/45] KVM: SVM: Do not use long-lived GHCB map while setting scratch area

Message ID 20210820155918.7518-35-brijesh.singh@amd.com (mailing list archive)
State Not Applicable
Delegated to: Herbert Xu
Headers show
Series Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support | expand

Commit Message

Brijesh Singh Aug. 20, 2021, 3:59 p.m. UTC 
The setup_vmgexit_scratch() function may rely on a long-lived GHCB
mapping if the GHCB shared buffer area was used for the scratch area.
In preparation for eliminating the long-lived GHCB mapping, always
allocate a buffer for the scratch area so it can be accessed without
the GHCB mapping.

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
---
 arch/x86/kvm/svm/sev.c | 70 +++++++++++++++++++-----------------------
 arch/x86/kvm/svm/svm.h |  3 +-
 2 files changed, 34 insertions(+), 39 deletions(-)

Comments

Sean Christopherson Oct. 13, 2021, 9:20 p.m. UTC | #1 
On Fri, Aug 20, 2021, Brijesh Singh wrote:
> The setup_vmgexit_scratch() function may rely on a long-lived GHCB
> mapping if the GHCB shared buffer area was used for the scratch area.
> In preparation for eliminating the long-lived GHCB mapping, always
> allocate a buffer for the scratch area so it can be accessed without
> the GHCB mapping.

Would it make sense to post this patch and the next (Remove the long-lived GHCB
host map) in a separate mini-series?  It's needed for SNP, but AFAICT there's
nothing that depends on SNP.  Getting this merged ahead of time would reduce the
size of the SNP series by a smidge.
Brijesh Singh Oct. 15, 2021, 4:11 p.m. UTC | #2 
On 10/13/21 2:20 PM, Sean Christopherson wrote:
> On Fri, Aug 20, 2021, Brijesh Singh wrote:
>> The setup_vmgexit_scratch() function may rely on a long-lived GHCB
>> mapping if the GHCB shared buffer area was used for the scratch area.
>> In preparation for eliminating the long-lived GHCB mapping, always
>> allocate a buffer for the scratch area so it can be accessed without
>> the GHCB mapping.
> Would it make sense to post this patch and the next (Remove the long-lived GHCB
> host map) in a separate mini-series?  It's needed for SNP, but AFAICT there's
> nothing that depends on SNP.  Getting this merged ahead of time would reduce the
> size of the SNP series by a smidge.

While testing with random configs, I am seeing some might_sleep() warns.
This is happening mainly because during the vmrun the GHCB is accessed
with preempt disabled. The kvm_vcpu_map() -> kmap() reports the warning.
I am leaning towards creating a cache on the vmgexit and use that cache
instead of the doing a kmap() on every access. Does that sound okay to you ?

-Brijesh
Sean Christopherson Oct. 15, 2021, 4:44 p.m. UTC | #3 
On Fri, Oct 15, 2021, Brijesh Singh wrote:
> 
> On 10/13/21 2:20 PM, Sean Christopherson wrote:
> > On Fri, Aug 20, 2021, Brijesh Singh wrote:
> >> The setup_vmgexit_scratch() function may rely on a long-lived GHCB
> >> mapping if the GHCB shared buffer area was used for the scratch area.
> >> In preparation for eliminating the long-lived GHCB mapping, always
> >> allocate a buffer for the scratch area so it can be accessed without
> >> the GHCB mapping.
> > Would it make sense to post this patch and the next (Remove the long-lived GHCB
> > host map) in a separate mini-series?  It's needed for SNP, but AFAICT there's
> > nothing that depends on SNP.  Getting this merged ahead of time would reduce the
> > size of the SNP series by a smidge.
> 
> While testing with random configs, I am seeing some might_sleep() warns.
> This is happening mainly because during the vmrun the GHCB is accessed
> with preempt disabled. The kvm_vcpu_map() -> kmap() reports the warning.
> I am leaning towards creating a cache on the vmgexit and use that cache
> instead of the doing a kmap() on every access. Does that sound okay to you ?

Since SEV is 64-bit only, it should be ok to add a kvm_vcpu_map_atomic() variant.
diff mbox series

Patch

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 2ad186d7e7b0..7dfb68e06334 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2490,8 +2490,7 @@  void sev_free_vcpu(struct kvm_vcpu *vcpu)
 	__free_page(virt_to_page(svm->vmsa));
 
 skip_vmsa_free:
-	if (svm->ghcb_sa_free)
-		kfree(svm->ghcb_sa);
+	kfree(svm->ghcb_sa);
 }
 
 static void dump_ghcb(struct vcpu_svm *svm)
@@ -2579,6 +2578,9 @@  static void sev_es_sync_from_ghcb(struct vcpu_svm *svm)
 	control->exit_info_1 = ghcb_get_sw_exit_info_1(ghcb);
 	control->exit_info_2 = ghcb_get_sw_exit_info_2(ghcb);
 
+	/* Copy the GHCB scratch area GPA */
+	svm->ghcb_sa_gpa = ghcb_get_sw_scratch(ghcb);
+
 	/* Clear the valid entries fields */
 	memset(ghcb->save.valid_bitmap, 0, sizeof(ghcb->save.valid_bitmap));
 }
@@ -2714,22 +2716,12 @@  void sev_es_unmap_ghcb(struct vcpu_svm *svm)
 	if (!svm->ghcb)
 		return;
 
-	if (svm->ghcb_sa_free) {
-		/*
-		 * The scratch area lives outside the GHCB, so there is a
-		 * buffer that, depending on the operation performed, may
-		 * need to be synced, then freed.
-		 */
-		if (svm->ghcb_sa_sync) {
-			kvm_write_guest(svm->vcpu.kvm,
-					ghcb_get_sw_scratch(svm->ghcb),
-					svm->ghcb_sa, svm->ghcb_sa_len);
-			svm->ghcb_sa_sync = false;
-		}
-
-		kfree(svm->ghcb_sa);
-		svm->ghcb_sa = NULL;
-		svm->ghcb_sa_free = false;
+	 /* Sync the scratch buffer area. */
+	if (svm->ghcb_sa_sync) {
+		kvm_write_guest(svm->vcpu.kvm,
+				ghcb_get_sw_scratch(svm->ghcb),
+				svm->ghcb_sa, svm->ghcb_sa_len);
+		svm->ghcb_sa_sync = false;
 	}
 
 	trace_kvm_vmgexit_exit(svm->vcpu.vcpu_id, svm->ghcb);
@@ -2767,12 +2759,11 @@  void pre_sev_run(struct vcpu_svm *svm, int cpu)
 static bool setup_vmgexit_scratch(struct vcpu_svm *svm, bool sync, u64 len)
 {
 	struct vmcb_control_area *control = &svm->vmcb->control;
-	struct ghcb *ghcb = svm->ghcb;
 	u64 ghcb_scratch_beg, ghcb_scratch_end;
 	u64 scratch_gpa_beg, scratch_gpa_end;
 	void *scratch_va;
 
-	scratch_gpa_beg = ghcb_get_sw_scratch(ghcb);
+	scratch_gpa_beg = svm->ghcb_sa_gpa;
 	if (!scratch_gpa_beg) {
 		pr_err("vmgexit: scratch gpa not provided\n");
 		return false;
@@ -2802,9 +2793,6 @@  static bool setup_vmgexit_scratch(struct vcpu_svm *svm, bool sync, u64 len)
 			       scratch_gpa_beg, scratch_gpa_end);
 			return false;
 		}
-
-		scratch_va = (void *)svm->ghcb;
-		scratch_va += (scratch_gpa_beg - control->ghcb_gpa);
 	} else {
 		/*
 		 * The guest memory must be read into a kernel buffer, so
@@ -2815,29 +2803,35 @@  static bool setup_vmgexit_scratch(struct vcpu_svm *svm, bool sync, u64 len)
 			       len, GHCB_SCRATCH_AREA_LIMIT);
 			return false;
 		}
+	}
+
+	if (svm->ghcb_sa_alloc_len < len) {
 		scratch_va = kzalloc(len, GFP_KERNEL_ACCOUNT);
 		if (!scratch_va)
 			return false;
 
-		if (kvm_read_guest(svm->vcpu.kvm, scratch_gpa_beg, scratch_va, len)) {
-			/* Unable to copy scratch area from guest */
-			pr_err("vmgexit: kvm_read_guest for scratch area failed\n");
-
-			kfree(scratch_va);
-			return false;
-		}
-
 		/*
-		 * The scratch area is outside the GHCB. The operation will
-		 * dictate whether the buffer needs to be synced before running
-		 * the vCPU next time (i.e. a read was requested so the data
-		 * must be written back to the guest memory).
+		 * Free the old scratch area and switch to using newly
+		 * allocated.
 		 */
-		svm->ghcb_sa_sync = sync;
-		svm->ghcb_sa_free = true;
+		kfree(svm->ghcb_sa);
+
+		svm->ghcb_sa_alloc_len = len;
+		svm->ghcb_sa = scratch_va;
 	}
 
-	svm->ghcb_sa = scratch_va;
+	if (kvm_read_guest(svm->vcpu.kvm, scratch_gpa_beg, svm->ghcb_sa, len)) {
+		/* Unable to copy scratch area from guest */
+		pr_err("vmgexit: kvm_read_guest for scratch area failed\n");
+		return false;
+	}
+
+	/*
+	 * The operation will dictate whether the buffer needs to be synced
+	 * before running the vCPU next time (i.e. a read was requested so
+	 * the data must be written back to the guest memory).
+	 */
+	svm->ghcb_sa_sync = sync;
 	svm->ghcb_sa_len = len;
 
 	return true;
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 27c0c7b265b8..85c852bb548a 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -199,8 +199,9 @@  struct vcpu_svm {
 	/* SEV-ES scratch area support */
 	void *ghcb_sa;
 	u64 ghcb_sa_len;
+	u64 ghcb_sa_gpa;
+	u32 ghcb_sa_alloc_len;
 	bool ghcb_sa_sync;
-	bool ghcb_sa_free;
 
 	bool guest_state_loaded;
 };