linux-kernel - Re: [PATCH RFC 2/7] kvm: x86: Introduce XFD MSRs as passthrough to guest

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Date:   Mon, 24 May 2021 21:43:00 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jing Liu <jing2.liu@...ux.intel.com>
Cc:     pbonzini@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, jing2.liu@...el.com
Subject: Re: [PATCH RFC 2/7] kvm: x86: Introduce XFD MSRs as passthrough to
 guest

On Sun, Feb 07, 2021, Jing Liu wrote:
> Passthrough both MSRs to let guest access and write without vmexit.

Why?  Except for read-only MSRs, e.g. MSR_CORE_C1_RES, passthrough MSRs are
costly to support because KVM must context switch the MSR (which, by the by, is
completely missing from the patch).

In other words, if these MSRs are full RW passthrough, guests with XFD enabled
will need to load the guest value on entry, save the guest value on exit, and
load the host value on exit.  That's in the neighborhood of a 40% increase in
latency for a single VM-Enter/VM-Exit roundtrip (~1500 cycles => >2000 cycles).

I'm not saying these can't be passhthrough, but there needs to be strong
justification for letting the guest read/write them directly.