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SubjectRe: [PATCH v6 02/25] x86/fpu/xstate: Fix guest fpstate allocation size calculation
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On 10/21/2023 8:39 AM, Sean Christopherson wrote:
> On Thu, Sep 14, 2023, Yang Weijiang wrote:
>> Fix guest xsave area allocation size from fpu_user_cfg.default_size to
>> fpu_kernel_cfg.default_size so that the xsave area size is consistent
>> with fpstate->size set in __fpstate_reset().
>>
>> With the fix, guest fpstate size is sufficient for KVM supported guest
>> xfeatures.
>>
>> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
>> ---
>> arch/x86/kernel/fpu/core.c | 4 +++-
>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
>> index a86d37052a64..a42d8ad26ce6 100644
>> --- a/arch/x86/kernel/fpu/core.c
>> +++ b/arch/x86/kernel/fpu/core.c
>> @@ -220,7 +220,9 @@ bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu)
>> struct fpstate *fpstate;
>> unsigned int size;
>>
>> - size = fpu_user_cfg.default_size + ALIGN(offsetof(struct fpstate, regs), 64);
>> + size = fpu_kernel_cfg.default_size +
>> + ALIGN(offsetof(struct fpstate, regs), 64);
> Shouldn't all the other calculations in this function also switch to fpu_kernel_cfg?
> At the very least, this looks wrong when paired with the above:
>
> gfpu->uabi_size = sizeof(struct kvm_xsave);
> if (WARN_ON_ONCE(fpu_user_cfg.default_size > gfpu->uabi_size))
> gfpu->uabi_size = fpu_user_cfg.default_size;

Hi, Sean,
Not sure what's your concerns.
From my understanding fpu_kernel_cfg.default_size should include all enabled xfeatures in host (XCR0 | XSS),
this is also expected for supporting all guest enabled xfeatures. gfpu->uabi_size only includes enabled user
xfeatures which are operated via KVM uABIs(KVM_GET_XSAVE/KVM_SET_XSAVE/KVM_GET_XSAVE2), so the two
sizes are relatively independent since guest supervisor xfeatures are saved/restored via GET/SET_MSRS interfaces.


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