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SubjectRe: [PATCH RFC 4/7] kvm: x86: Add new ioctls for XSAVE extension
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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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