Linux Virtualization - Mar 2023 - [PATCH] x86/paravirt: convert simple paravirt functions to asm

On Wed, Mar 08, 2023 at 04:42:10PM +0100, Juergen Gross
wrote:
> All functions referenced via __PV_IS_CALLEE_SAVE() need to be assembler
> functions, as those functions calls are hidden from gcc. In case the
> kernel is compiled with "-fzero-call-used-regs" the compiler will
> clobber caller-saved registers at the end of C functions, which will
> result in unexpectedly zeroed registers at the call site of the
> related paravirt functions.
> 
> Replace the C functions with DEFINE_PARAVIRT_ASM() constructs using
> the same instructions as the related paravirt calls in the
> PVOP_ALT_[V]CALLEE*() macros.
> 
> Signed-off-by: Juergen Gross <jgross at suse.com>
> ---
>  arch/x86/include/asm/paravirt_types.h |  8 +++++++-
>  arch/x86/kernel/paravirt.c            | 27 ++++++---------------------
>  2 files changed, 13 insertions(+), 22 deletions(-)
Right, works with my particular reproducer.

Turning them into asm prevents the compiler from doing the
callee-clobbered zeroing and that's fine as this whole paravirt gunk is
hiding the "CALL" insn from it and you putting them in asm is in line
with this.

And a negative diffstat..

So yeah, I'll queue it soon unless someone objects.

Long term, I think we should continue switching all that pv stuff to
using the alternatives.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

On 09.03.23 14:39, Borislav Petkov wrote:
> On Wed, Mar 08, 2023 at 04:42:10PM +0100, Juergen Gross wrote:
>> All functions referenced via __PV_IS_CALLEE_SAVE() need to be assembler
>> functions, as those functions calls are hidden from gcc. In case the
>> kernel is compiled with "-fzero-call-used-regs" the compiler
will
>> clobber caller-saved registers at the end of C functions, which will
>> result in unexpectedly zeroed registers at the call site of the
>> related paravirt functions.
>>
>> Replace the C functions with DEFINE_PARAVIRT_ASM() constructs using
>> the same instructions as the related paravirt calls in the
>> PVOP_ALT_[V]CALLEE*() macros.
>>
>> Signed-off-by: Juergen Gross <jgross at suse.com>
>> ---
>>   arch/x86/include/asm/paravirt_types.h |  8 +++++++-
>>   arch/x86/kernel/paravirt.c            | 27
++++++---------------------
>>   2 files changed, 13 insertions(+), 22 deletions(-)
> 
> Right, works with my particular reproducer.
> 
> Turning them into asm prevents the compiler from doing the
> callee-clobbered zeroing and that's fine as this whole paravirt gunk is
> hiding the "CALL" insn from it and you putting them in asm is in
line
> with this.
> 
> And a negative diffstat..
> 
> So yeah, I'll queue it soon unless someone objects.
Thanks.
> Long term, I think we should continue switching all that pv stuff to
> using the alternatives.
The "normal" cases not using alternatives should rather be switched to
static calls.

Whether it is possible to mix a static call with alternatives needs to
be evaluated.


Juergen
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