Re: [PATCH 4/4] x86/xen: drop USERGS_SYSRET64 paravirt call

[Jürgen Groß <jgross@xxxxxxxx>]

On 16.11.20 17:28, Andy Lutomirski wrote:
> On Mon, Nov 16, 2020 at 7:23 AM Juergen Gross <jgross@xxxxxxxx> wrote:
> > USERGS_SYSRET64 is used to return from a syscall via sysret, but
> > a Xen PV guest will nevertheless use the iret hypercall, as there
> > is no sysret PV hypercall defined.
> >
> > So instead of testing all the prerequisites for doing a sysret and
> > then mangling the stack for Xen PV again for doing an iret just use
> > the iret exit from the beginning.
> >
> > This can easily be done via an ALTERNATIVE like it is done for the
> > sysenter compat case already.
> >
> > While at it remove to stale sysret32 remnants.
> >
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> Acked-by: Andy Lutomirski <luto@xxxxxxxxxx>
>
> FWIW, you've lost the VGCF_in_syscall optimization.  Let me see if I
> can give it back to you better.
Ah, right.

Nevertheless a simple kernel build is about 0.5% faster with this
patch.


Juergen

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