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    SubjectRe: [PATCH 4/4] x86/xen: drop USERGS_SYSRET64 paravirt call
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    On 16.11.20 17:28, Andy Lutomirski wrote:
    > On Mon, Nov 16, 2020 at 7:23 AM Juergen Gross <jgross@suse.com> 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@suse.com>
    >
    > Acked-by: Andy Lutomirski <luto@kernel.org>
    >
    > 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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