linux-kernel - Re: [PATCH] x86/paravirt: convert simple paravirt functions to asm

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Date:   Thu, 9 Mar 2023 14:39:50 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Juergen Gross <jgross@...e.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        virtualization@...ts.linux-foundation.org,
        "Srivatsa S. Bhat (VMware)" <srivatsa@...il.mit.edu>,
        Alexey Makhalov <amakhalov@...are.com>,
        VMware PV-Drivers Reviewers <pv-drivers@...are.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [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@...e.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