linux-kernel - Re: [PATCH v10 10/28] x86/fpu/xstate: Update the XSTATE save function to support dynamic states

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Date:   Tue, 05 Oct 2021 11:55:56 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>, bp@...e.de,
        luto@...nel.org, mingo@...nel.org, x86@...nel.org
Cc:     len.brown@...el.com, lenb@...nel.org, dave.hansen@...el.com,
        thiago.macieira@...el.com, jing2.liu@...el.com,
        ravi.v.shankar@...el.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v10 10/28] x86/fpu/xstate: Update the XSTATE save
 function to support dynamic states

On Tue, Oct 05 2021 at 09:50, Paolo Bonzini wrote:
> On 02/10/21 23:31, Thomas Gleixner wrote:
>> You have two options:
>> 
>>    1) Always allocate the large buffer size which is required to
>>       accomodate all possible features.
>> 
>>       Trivial, but waste of memory.
>> 
>>    2) Make the allocation dynamic which seems to be trivial to do in
>>       kvm_load_guest_fpu() at least for vcpu->user_fpu.
>> 
>>       The vcpu->guest_fpu handling can probably be postponed to the
>>       point where AMX is actually exposed to guests, but it's probably
>>       not the worst idea to think about the implications now.
>> 
>> Paolo, any opinions?
>
> Unless we're missing something, dynamic allocation should not be hard to 
> do for both guest_fpu and user_fpu; either near the call sites of 
> kvm_save_current_fpu, or in the function itself.  Basically adding 
> something like
>
> 	struct kvm_fpu {
> 		struct fpu *state;
> 		unsigned size;
> 	} user_fpu, guest_fpu;
>
> to struct kvm_vcpu.  Since the size can vary, it can be done simply with 
> kzalloc instead of the x86_fpu_cache that KVM has now.
>
> The only small complication is that kvm_save_current_fpu is called 
> within fpregs_lock; the allocation has to be outside so that you can use 
> GFP_KERNEL even on RT kernels.   If the code looks better with 
> fpregs_lock moved within kvm_save_current_fpu, go ahead and do it like that.

I'm reworking quite some of this already and with the new bits you don't
have to do anything in kvm_fpu because the size and allowed feature bits
are going to be part of fpu->fpstate.

Stay tuned.

Thanks,

        tglx