linux-kernel - Re: [PATCH 16/19] kvm: x86: Introduce KVM_{G|S}ET

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Message-ID: <d16aab21-0f81-f758-a61e-5919f223be78@redhat.com>
Date:   Fri, 10 Dec 2021 17:30:06 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Yang Zhong <yang.zhong@...el.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com
Cc:     seanjc@...gle.com, jun.nakajima@...el.com, kevin.tian@...el.com,
        jing2.liu@...ux.intel.com, jing2.liu@...el.com
Subject: Re: [PATCH 16/19] kvm: x86: Introduce KVM_{G|S}ET_XSAVE2 ioctl

On 12/8/21 01:03, Yang Zhong wrote:
> +static void kvm_vcpu_ioctl_x86_get_xsave2(struct kvm_vcpu *vcpu,
> +					  u8 *state, u32 size)
> +{
> +	if (fpstate_is_confidential(&vcpu->arch.guest_fpu))
> +		return;
> +
> +	fpu_copy_guest_fpstate_to_uabi(&vcpu->arch.guest_fpu,
> +				       state, size,
> +				       vcpu->arch.pkru);
> +}
> +
>   static int kvm_vcpu_ioctl_x86_set_xsave(struct kvm_vcpu *vcpu,
>   					struct kvm_xsave *guest_xsave)
>   {
> @@ -4951,6 +4963,15 @@ static int kvm_vcpu_ioctl_x86_set_xsave(struct kvm_vcpu *vcpu,
>   					      supported_xcr0, &vcpu->arch.pkru);
>   }
>   
> +static int kvm_vcpu_ioctl_x86_set_xsave2(struct kvm_vcpu *vcpu, u8 *state)
> +{
> +	if (fpstate_is_confidential(&vcpu->arch.guest_fpu))
> +		return 0;
> +
> +	return fpu_copy_uabi_to_guest_fpstate(&vcpu->arch.guest_fpu, state,
> +					      supported_xcr0, &vcpu->arch.pkru);
> +}
> +

I think fpu_copy_uabi_to_guest_fpstate (and therefore 
copy_uabi_from_kernel_to_xstate) needs to check that the size is 
compatible with the components in the input.

Also, IIUC the size of the AMX state will vary in different processors. 
  Is this correct?  If so, this should be handled already by 
KVM_GET/SET_XSAVE2 and therefore should be part of the 
arch/x86/kernel/fpu APIs.  In the future we want to support migrating a 
"small AMX" host to a "large AMX" host; and also migrating from a "large 
AMX" host to a "small AMX" host if the guest CPUID is compatible with 
the destination of the migration.

Paolo