Re: [PATCH] x86: vmx: Allow direct access to MSR_IA32_SPEC_CTRL

[Liran Alon <liran.alon@oracle.com>]

----- dwmw2@infradead.org wrote:

> On Sun, 2018-01-28 at 15:21 -0500, Konrad Rzeszutek Wilk wrote:
> > >To avoid the overhead of atomically saving and restoring the
> MSR_IA32_SPEC_CTRL
> > >for guests that do not actually use the MSR, only
> add_atomic_switch_msr when a
> > >non-zero is written to it.
> > 
> > 
> > We tried this and found that it was about 3% slower that doing the
> > old way of rdmsr and wrmsr.
> > 
> > But that was also with the host doing IBRS  as well.
> 
> The common case will be that neither host nor guest are doing IBRS.
> Until the guest touches the MSR we do absolutely *nothing* with it,
> which is definitely the fastest option.

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?

-Liran