| Message ID | 20241126101710.62492-1-chao.gao@intel.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | Introduce CET supervisor state support | expand |
On Tue, Nov 26, 2024 at 06:17:04PM +0800, Chao Gao wrote: >This v2 is essentially a resend of the v1 series. I took over this work >from Weijiang, so I added my Signed-off-by and incremented the version >number. This repost is to seek more feedback on this work, which is a >dependency for CET KVM support. In turn, CET KVM support is a dependency >for both FRED KVM support and CET AMD support. This series is primarily for the CET KVM series. Merging it through the tip tree means this code will not have an actual user until the CET KVM series is merged. A good proposal from Rick is that x86 maintainers can ack this series, and then it can be picked up by the KVM maintainers along with the CET KVM series. Dave, Paolo and Sean, are you okay with this approach?
This v2 is essentially a resend of the v1 series. I took over this work from Weijiang, so I added my Signed-off-by and incremented the version number. This repost is to seek more feedback on this work, which is a dependency for CET KVM support. In turn, CET KVM support is a dependency for both FRED KVM support and CET AMD support. ==Background== This series spins off from CET KVM virtualization enabling series [1]. The purpose is to get these preparation work resolved ahead of KVM part landing. There was a discussion about introducing CET supervisor state support [2] [3]. CET supervisor state, i.e., IA32_PL{0,1,2}_SSP, are xsave-managed MSRs, it can be enabled via IA32_XSS[bit 12]. KVM relies on host side CET supervisor state support to fully enable guest CET MSR contents storage. The benefits are: 1) No need to manually save/restore the 3 MSRs when vCPU fpu context is sched in/out. 2) Omit manually swapping the three MSRs at VM-Exit/VM-Entry for guest/host. 3) Make guest CET user/supervisor states managed in a consistent manner within host kernel FPU framework. ==Solution== This series tries to: 1) Fix existing issue regarding enabling guest supervisor states support. 2) Add CET supervisor state support in core kernel. 3) Introduce new FPU config for guest fpstate setup. With the preparation work landed, for guest fpstate, we have xstate_bv[12] == xcomp_bv[12] == 1 and CET supervisor state is saved/reloaded when xsaves/xrstors executes on guest fpstate. For non-guest/normal fpstate, we have xstate_bv[12] == xcomp_bv[12] == 0, then HW can optimize xsaves/xrstors operations. ==Performance== We measured context-switching performance with the benchmark [4] in following three cases. case 1: the baseline. i.e., this series isn't applied case 2: baseline + this series. CET-S space is allocated for guest fpu only. case 3: baseline + allocate CET-S space for all tasks. Hardware init optimization avoids writing out CET-S space on each XSAVES. the data are as follows case |IA32_XSS[12] | Space | RFBM[12] | Drop% -----+-------------+-------+----------+------ 1 | 0 | None | 0 | 0.0% 2 | 1 | None | 0 | 0.2% 3 | 1 | 24B? | 1 | 0.2% Case 2 and 3 have no difference in performnace. But case 2 is preferred because it can save 24B of CET-S space for all non-vCPU threads with just a one-line change in patch 3: + fpu_kernel_cfg.default_features &= ~XFEATURE_MASK_KERNEL_DYNAMIC; Patch 4 and 5 have their own merits. Regardless of the approach we take, using different FPU configuration settings for the guest and the kernel improves readability, decouples them from each other, and arguably enhances extensibility. ==Changelog== v1->v2: - rebase onto the latest kvm-x86/next - Add performance data to the cover-letter - v1: https://lore.kernel.org/kvm/73802bff-833c-4233-9a5b-88af0d062c82@intel.com/ ==Organization== Patch1: Preserve guest supervisor xfeatures in __state_perm. Patch2: Enable CET supervisor xstate support. Patch3: Introduce kernel dynamic xfeature set. Patch4: Initialize fpu_guest_cfg settings. Patch5: Create guest fpstate with fpu_guest_cfg. Patch6: Check invalid fpstate config before executes xsaves. [1]: https://lore.kernel.org/all/20240219074733.122080-1-weijiang.yang@intel.com/ [2]: https://lore.kernel.org/all/ZM1jV3UPL0AMpVDI@google.com/ [3]: https://lore.kernel.org/all/2597a87b-1248-b8ce-ce60-94074bc67ea4@intel.com/ [4]: https://github.com/antonblanchard/will-it-scale/blob/master/tests/context_switch1.c Sean Christopherson (1): x86/fpu/xstate: Always preserve non-user xfeatures/flags in __state_perm Yang Weijiang (5): x86/fpu/xstate: Add CET supervisor mode state support x86/fpu/xstate: Introduce XFEATURE_MASK_KERNEL_DYNAMIC xfeature set x86/fpu/xstate: Introduce fpu_guest_cfg for guest FPU configuration x86/fpu/xstate: Create guest fpstate with guest specific config x86/fpu/xstate: Warn if CET supervisor state is detected in normal fpstate arch/x86/include/asm/fpu/types.h | 16 ++++++++-- arch/x86/include/asm/fpu/xstate.h | 11 ++++--- arch/x86/kernel/fpu/core.c | 53 ++++++++++++++++++++++++------- arch/x86/kernel/fpu/xstate.c | 35 +++++++++++++++----- arch/x86/kernel/fpu/xstate.h | 2 ++ 5 files changed, 90 insertions(+), 27 deletions(-)