linux-kernel - Re: [PATCH 12/19] x86/fpu: Prepare KVM for bringing XFD state back in-sync

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Message-ID: <87wnkcvyv8.ffs@tglx>
Date:   Sat, 11 Dec 2021 00:11:55 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     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,
        pbonzini@...hat.com
Cc:     seanjc@...gle.com, jun.nakajima@...el.com, kevin.tian@...el.com,
        jing2.liu@...ux.intel.com, jing2.liu@...el.com,
        yang.zhong@...el.com
Subject: Re: [PATCH 12/19] x86/fpu: Prepare KVM for bringing XFD state back
 in-sync

On Tue, Dec 07 2021 at 19:03, Yang Zhong wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
>
> Guest may toggle IA32_XFD in high frequency as it is part of the fpstate
> information (features, sizes, xfd) and swapped in task context switch.
>
> To minimize the trap overhead of writes to this MSR, one optimization
> is to allow guest direct write thus eliminate traps. However MSR
> passthrough implies that guest_fpstate::xfd and per-cpu xfd cache might
> be out of sync with the current IA32_XFD value by the guest.
>
> This suggests KVM needs to re-sync guest_fpstate::xfd and per-cpu cache
> with IA32_XFD before the vCPU thread might be preempted or interrupted.
>
> This patch provides a helper function for the re-sync purpose.

Provide a ....

> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Jing Liu <jing2.liu@...el.com>
> Signed-off-by: Yang Zhong <yang.zhong@...el.com>
> ---
> (To Thomas): the original name kvm_update_guest_xfd_state() in
> your sample code is renamed to xfd_sync_state() in this patch. In
> concept it is a general helper to bring software values in-sync with
> the MSR value after they become out-of-sync. KVM is just the
> first out-of-sync usage on this helper, so a neutral name may make
> more sense. But if you prefer to the original name we can also
> change back.

There is no need for a general helper, really.

It's KVM specific and should go into KVM section in core.c next to the
other thing vs. the XFD update.

Thanks,

        tglx