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

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Message-ID: <87bl1kvmqg.ffs@tglx>
Date:   Mon, 13 Dec 2021 11:10:47 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Yang Zhong <yang.zhong@...el.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        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 Fri, Dec 10 2021 at 17:30, Paolo Bonzini wrote:
> On 12/8/21 01:03, Yang Zhong wrote:
>> +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.

fpu_copy_uabi_to_guest_fpstate() expects that the input buffer is
correctly sized. We surely can add a size check there.

> 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.

How is that supposed to work? If the AMX state size differs then the
hosts are not compatible.

Thanks,

        tglx