linux-kernel - Re: [RFC PATCH 3/3] x86/paravirt: switch mixed paravirt/alternative calls to alternative

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Date:   Wed, 20 Sep 2023 16:52:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juergen Gross <jgross@...e.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        virtualization@...ts.linux-foundation.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Srivatsa S. Bhat (VMware)" <srivatsa@...il.mit.edu>,
        Alexey Makhalov <amakhalov@...are.com>,
        VMware PV-Drivers Reviewers <pv-drivers@...are.com>
Subject: Re: [RFC PATCH 3/3] x86/paravirt: switch mixed paravirt/alternative
 calls to alternative_2

On Thu, Jun 08, 2023 at 04:03:33PM +0200, Juergen Gross wrote:
> Instead of stacking alternative and paravirt patching, use the new
> ALT_FLAG_CALL flag to switch those mixed calls to pure alternative
> handling.
> 
> This eliminates the need to be careful regarding the sequence of
> alternative and paravirt patching.
> 
> For call depth tracking callthunks_setup() needs to be adapted to patch
> calls at alternative patching sites instead of paravirt calls.
> 
> Remove the no longer needed paravirt patching and related code.

I think this becomes easier if you first convert the paravirt sites to
alternatives, such that .parainstructions is empty, and then in a
subsequent patch remove all the paravirt infrastructure that is unused.


> +#define SAVE_FLAGS	ALTERNATIVE_2 "PARA_IRQ_save_fl;", ALT_CALL_INSTR, \
> +				      ALT_CALL_ALWAYS, "pushf; pop %rax;", \
> +				      ALT_NOT(X86_FEATURE_XENPV)

I find this more readable when written as:

#define SAVE_FLAGS	ALTERNATIVE_2 "PARA_IRQ_save_fl;",		\
				      ALT_CALL_INSTR, ALT_CALL_ALWAYS,	\
				      "pushf; pop %rax;", ALT_NOT(X86_FEATURE_XENPV)

(and perhaps ALT_NOT_XEN is in order, there's a ton of those)

If you base this on top of the nested alternative patches, another
helper might be:

#define __PV_ALTERNATIVE(old) __ALTERNATIVE(old, ALT_CALL_INSTR, ALT_CALL_ALWAYS)

So that you can then write:

#define SAVE_FLAGS	__ALTERNATIVE(__PV_ALTERNATIVE("PARA_IRQ_save_fl;"),
				      "pushf; pop %rax;", ALT_NOT_XEN)

But perhaps I'm over-cooking things now..