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

["Jürgen Groß" <jgross@suse.com>]

[-- Attachment #1.1.1: Type: text/plain, Size: 969 bytes --]

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

[-- Attachment #1.1.2: OpenPGP_0xB0DE9DD628BF132F.asc --]
[-- Type: application/pgp-keys, Size: 3135 bytes --]

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 495 bytes --]