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SubjectRe: [PATCH v3 16/22] kvm: x86: Add XCR0 support for Intel AMX
On Wed, Dec 22, 2021, Jing Liu wrote:
> Two XCR0 bits are defined for AMX to support XSAVE mechanism. Bit 17
> is for tilecfg and bit 18 is for tiledata.
>
> The value of XCR0[17:18] is always either 00b or 11b.

Is that an SDM requirement, or an arbitrary Linux/KVM requirement?

> Also, SDM
> recommends that only 64-bit operating systems enable Intel AMX by
> setting XCR0[18:17]. If a 32-bit guest tries to set dynamic bits, it

This is wrong. It has nothing to do with 32-bit guests. The restriction is on
32-bit _host kernels_, which I'm guessing never set the tile bits in _host_ XCR0.

> fails to pass vcpu->arch.guest_supported_xcr0 check and gets a #GP.
>
> Signed-off-by: Yang Zhong <yang.zhong@intel.com>
> Signed-off-by: Jing Liu <jing2.liu@intel.com>
> ---
> arch/x86/kvm/x86.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a48a89f73027..c558c098979a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -210,7 +210,7 @@ static struct kvm_user_return_msrs __percpu *user_return_msrs;
> #define KVM_SUPPORTED_XCR0 (XFEATURE_MASK_FP | XFEATURE_MASK_SSE \
> | XFEATURE_MASK_YMM | XFEATURE_MASK_BNDREGS \
> | XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
> - | XFEATURE_MASK_PKRU)
> + | XFEATURE_MASK_PKRU | XFEATURE_MASK_XTILE)
>
> u64 __read_mostly host_efer;
> EXPORT_SYMBOL_GPL(host_efer);
> @@ -990,6 +990,12 @@ static int __kvm_set_xcr(struct kvm_vcpu *vcpu, u32 index, u64 xcr)
> if ((xcr0 & XFEATURE_MASK_AVX512) != XFEATURE_MASK_AVX512)
> return 1;
> }
> +
> +#ifdef CONFIG_X86_64

Drop the #ifdef, it adds no meaningful value and requires the reader to think
far harder than they should have. Yes, it's technically dead code for 32-bit KVM,
but no one cares about performance of 32-bit KVM, and in any case it's extremely
unlikely this will be anything but noise.

> + if ((xcr0 & XFEATURE_MASK_XTILE) &&
> + ((xcr0 & XFEATURE_MASK_XTILE) != XFEATURE_MASK_XTILE))
> + return 1;
> +#endif
> vcpu->arch.xcr0 = xcr0;
>
> if ((xcr0 ^ old_xcr0) & XFEATURE_MASK_EXTEND)
> --
> 2.27.0
>

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