Messages in this thread | | | Subject | Re: [PATCH RFC 4/7] kvm: x86: Add new ioctls for XSAVE extension | From | "Liu, Jing2" <> | Date | Wed, 26 May 2021 14:09:03 +0800 |
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On 5/25/2021 5:50 AM, Sean Christopherson wrote: > On Sun, Feb 07, 2021, Jing Liu wrote: >> The static xstate buffer kvm_xsave contains the extended register >> states, but it is not enough for dynamic features with large state. >> >> Introduce a new capability called KVM_CAP_X86_XSAVE_EXTENSION to >> detect if hardware has XSAVE extension (XFD). Meanwhile, add two >> new ioctl interfaces to get/set the whole xstate using struct >> kvm_xsave_extension buffer containing both static and dynamic >> xfeatures. Reuse fill_xsave and load_xsave for both cases. >> >> Signed-off-by: Jing Liu <jing2.liu@linux.intel.com> >> --- >> arch/x86/include/uapi/asm/kvm.h | 5 +++ >> arch/x86/kvm/x86.c | 70 +++++++++++++++++++++++++-------- >> include/uapi/linux/kvm.h | 8 ++++ >> 3 files changed, 66 insertions(+), 17 deletions(-) >> >> diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h >> index 89e5f3d1bba8..bf785e89a728 100644 >> --- a/arch/x86/include/uapi/asm/kvm.h >> +++ b/arch/x86/include/uapi/asm/kvm.h >> @@ -362,6 +362,11 @@ struct kvm_xsave { >> __u32 region[1024]; >> }; >> >> +/* for KVM_CAP_XSAVE_EXTENSION */ >> +struct kvm_xsave_extension { >> + __u32 region[3072]; > Fool me once, shame on you (Intel). Fool me twice, shame on me (KVM). > > As amusing as kvm_xsave_really_extended would be, the required size should be > discoverable, not hardcoded. Thanks for reviewing the patch. When looking at current kvm_xsave structure, I felt confusing about the static hardcoding of 1024 bytes, but failed to find clue for its final decision in 2010[1]. So we'd prefer to changing the way right? Please correct me if I misunderstood.
> Nothing prevents a hardware vendor from inventing > a newfangled feature that requires yet more space. > As an alternative to adding a dedicated capability, can we leverage > GET_SUPPORTED_CPUID, leaf CPUID.0xD, Yes, this is a good way to avoid a dedicated capability. Thanks for the suggestion. Use 0xD.1.EBX for size of enabled xcr0|xss if supposing kvm_xsave cares both. > to enumerate the minimum required size and > state For the state, an extreme case is using an old qemu as follows, but a new kvm with more future_featureZ supported. If hardware vendor arranges one by one, it's OK to use static state like X86XSaveArea(2) and get/set between userspace and kvm because it's non-compacted. If not, the state will not correct. So far it is OK, so I'm wondering if this would be an issue for now?
X86XSaveArea2 { ... XSaveAVX ... AMX_XTILE; future_featureX; future_featureY; }
> that the new ioctl() is available if the min size is greater than 1024? > Or is that unnecessarily convoluted... To enable a dynamic size kvm_xsave2(Thanks Jim's name suggestion), if things as follows are what we might want. /* for xstate large than 1024 */ struct kvm_xsave2 { int size; // size of the whole xstate void *ptr; // xstate pointer } #define KVM_GET_XSAVE2 _IOW(KVMIO, 0xa4, struct kvm_xsave2)
Take @size together, so KVM need not fetch 0xd.1.ebx each time or a dedicated variable.
For Userspace(Qemu): struct X86XSaveArea2 {...}// new struct holding all features
if 0xd.1.ebx <= sizeof(kvm_xsave) env->xsave_buf = alloc(sizeof(kvm_xsave)) ... ioctl(KVM_GET/SET_XSAVE, X86XSaveArea *) else env->xsave_buf = alloc(0xd.1.ebx + sizeof(int)) ... xsave2 = env->xsave_buf xsave2->size = ... X86XSaveArea2 *area2 = xsave2->ptr ioctl(KVM_GET/SET_XSAVE2, xsave2) endif
[1] https://lore.kernel.org/kvm/4C10AE1D.40604@redhat.com/
Thanks, Jing
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