linux-kernel - Re: [PATCH] x86: vmx: Allow direct access to MSR_IA32_SPEC

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Message-ID: <1517215563.6624.118.camel@infradead.org>
Date:   Mon, 29 Jan 2018 08:46:03 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Liran Alon <liran.alon@...cle.com>
Cc:     konrad.wilk@...cle.com, luto@...nel.org, tglx@...utronix.de,
        torvalds@...ux-foundation.org, gregkh@...uxfoundation.org,
        asit.k.mallick@...el.com, dave.hansen@...el.com,
        karahmed@...zon.de, jun.nakajima@...el.com,
        dan.j.williams@...el.com, ashok.raj@...el.com,
        daniel.kiper@...cle.com, arjan.van.de.ven@...el.com,
        tim.c.chen@...ux.intel.com, pbonzini@...hat.com,
        linux-kernel@...r.kernel.org, ak@...ux.intel.com,
        kvm@...r.kernel.org, aarcange@...hat.com
Subject: Re: [PATCH] x86: vmx: Allow direct access to MSR_IA32_SPEC_CTRL

On Sun, 2018-01-28 at 16:39 -0800, Liran Alon wrote:
> 
> Windows use IBRS and Microsoft don't have any plans to switch to retpoline.
> Running a Windows guest should be a pretty common use-case no?
> 
> In addition, your handle of the first WRMSR intercept could be different.
> It could signal you to start doing the following:
> 1. Disable intercept on SPEC_CTRL MSR.
> 2. On VMEntry, Write vCPU SPEC_CTRL value into physical MSR.
> 3. On VMExit, read physical MSR into vCPU SPEC_CTRL value.
> (And if IBRS is used at host, also set physical SPEC_CTRL MSR here to 1)
> 
> That way, you will both have fastest option as long as guest don't use IBRS
> and also won't have the 3% performance hit compared to Konrad's proposal.
> 
> Am I missing something?

Reads from the SPEC_CTRL MSR are strangely slow. I suspect a large part
of the 3% speedup you observe is because in the above, the vmentry path
doesn't need to *read* the host's value and store it; the host is
expected to restore it for itself anyway?

I'd actually quite like to repeat the benchmark on the new fixed
microcode, if anyone has it yet, to see if that read/swap slowness is
still quite as excessive. I'm certainly not ruling this out, but I'm
just a little wary of premature optimisation, and I'd like to make sure
we have everything *else* in the KVM patches right first.

The fact that the save-and-restrict macros I have in the tip of my
working tree at the moment are horrid and causing 0-day nastygrams,
probably doesn't help persuade me to favour the approach ;)

... hm, the CPU actually has separate MSR save/restore lists for
entry/exit, doesn't it? Is there any way to sanely make use of that and
do the restoration manually on vmentry but let it be automatic on
vmexit, by having it *only* in the guest's MSR-store area to be saved
on exit and restored on exit, but *not* in the host's MSR-store area?

Reading the code and comparing with the SDM, I can't see where we're
ever setting VM_EXIT_MSR_STORE_{ADDR,COUNT} except in the nested
case...
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