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

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Message-ID: <022620db-13ad-8118-5296-ae2913d41f1f@redhat.com>
Date:   Mon, 13 Dec 2021 16:06:22 +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

On 12/8/21 01:03, Yang Zhong wrote:
> +		/*
> +		 * Update IA32_XFD to the guest value so #NM can be
> +		 * raised properly in the guest. Instead of directly
> +		 * writing the MSR, call a helper to avoid breaking
> +		 * per-cpu cached value in fpu core.
> +		 */
> +		fpregs_lock();
> +		current->thread.fpu.fpstate->xfd = data;

This is wrong, it should be written in vcpu->arch.guest_fpu.

> +		xfd_update_state(current->thread.fpu.fpstate);

This is okay though, so that KVM_SET_MSR will not write XFD and WRMSR will.

That said, I think xfd_update_state should not have an argument. 
current->thread.fpu.fpstate->xfd is the only fpstate that should be 
synced with the xfd_state per-CPU variable.

Paolo

> +		fpregs_unlock();