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

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Date:   Mon, 13 Dec 2021 20:45:52 +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 10/19] kvm: x86: Emulate WRMSR of guest IA32_XFD

On Mon, Dec 13 2021 at 16:06, Paolo Bonzini wrote:
> 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.

I'm looking into this right now. The whole restore versus runtime thing
needs to be handled differently.

Thanks,

        tglx