linux-kernel - Re: [PATCH v6 02/25] x86/fpu/xstate: Fix guest fpstate allocation size calculation

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Date:   Tue, 24 Oct 2023 16:50:52 +0800
From:   "Yang, Weijiang" <weijiang.yang@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <dave.hansen@...el.com>,
        <peterz@...radead.org>, <chao.gao@...el.com>,
        <rick.p.edgecombe@...el.com>, <john.allen@....com>
Subject: Re: [PATCH v6 02/25] x86/fpu/xstate: Fix guest fpstate allocation
 size calculation

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@...el.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.