Messages in this thread |  | | Subject | Re: [PATCH RFC 4/7] kvm: x86: Add new ioctls for XSAVE extension | From | "Liu, Jing2" <> | Date | Tue, 1 Jun 2021 18:24:34 +0800 |
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On 5/26/2021 10:43 PM, Sean Christopherson wrote: > On Wed, May 26, 2021, Liu, Jing2 wrote: >> 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]; > Hold up a sec. How big is the AMX data? AMX tileconfig size is 64B, but tiledata size is 8K. > The existing size is 4096 bytes, not > 1024 bytes. IIRC, AMX is >4k, so we still need a new ioctl(), Yep, kvm_xsave can hold 4KB state. We need a new ioctl, holding all the states, not only AMX. And once KVM supports AMX, the size will >4096 so qemu need use kvm_xsave2 instead, otherwise, cannot save/restore whole AMX state. > but we should be > careful to mentally adjust for the __u32 when mentioning the sizes. > >>>> }; >>>> +/* 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]. > Simplicitly and lack of foresight :-) > >> So we'd prefer to changing the way right? Please correct me if I misunderstood. > Sadly, we can't fix the existing ioctl() without breaking userspace. But for > the new ioctl(), yes, its size should not be hardcoded. > >>> 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? > Oh, you're saying that, because kvm_xsave is non-compacted, future features may > overflow kvm_xsave simply because the architectural offset overflows 4096 bytes. > > That should be a non-issue for old KVM/kernels, since the new features shouldn't > be enabled. For new KVM, I think the right approach is to reject KVM_GET_XSAVE > and KVM_SET_XSAVE if the required size is greater than sizeof(struct kvm_xsave). > I.e. force userspace to either hide the features from the guest, or use > KVM_{G,S}ET_XSAVE2. I was considering if the order/offset of future features will impact the compatibility if it is not designed one by one. But I realized it's not an issue because there uses non-compacted format so each offset strictly refers to spec.
>> 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. > Yes, userspace needs to provide the size so that KVM doesn't unintentionally > overflow the buffer provided by userspace. We might also want to hedge by adding > a flags? Can't think of a use for it at the moment, though. > > struct kvm_xsave2 { > __u32 flags; > __u32 size; > __u8 state[0]; > }; u8 makes things simple that kvm doesn't need compute size to u32.
Thanks, Jing
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