linux-kernel - Re: [PATCH 10/19] kvm: x86: Emulate WRMSR of guest IA32

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Date:   Fri, 10 Dec 2021 17:02:49 +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 10/19] kvm: x86: Emulate WRMSR of guest IA32_XFD

First, the MSR should be added to msrs_to_save_all and 
kvm_cpu_cap_has(X86_FEATURE_XFD) should be checked in kvm_init_msr_list.

It seems that RDMSR support is missing, too.

More important, please include:

- documentation for the new KVM_EXIT_* value

- a selftest that explains how userspace should react to it.

This is a strong requirement for any new API (the first has been for 
years; but the latter is also almost always respected these days).  This 
series should not have been submitted without documentation.

Also:

On 12/8/21 01:03, Yang Zhong wrote:
> 
> +		if (!guest_cpuid_has(vcpu, X86_FEATURE_XFD))
> +			return 1;

This should allow msr->host_initiated always (even if XFD is not part of 
CPUID).  However, if XFD is nonzero and kvm_check_guest_realloc_fpstate 
returns true, then it should return 1.

The selftest should also cover using KVM_GET_MSR/KVM_SET_MSR.

> +		/* Setting unsupported bits causes #GP */
> +		if (~XFEATURE_MASK_USER_DYNAMIC & data) {
> +			kvm_inject_gp(vcpu, 0);
> +			break;
> +		}

This should check

	if (data & ~(XFEATURE_MASK_USER_DYNAMIC &
		    vcpu->arch.guest_supported_xcr0))

instead.

Paolo