Re: [PATCH v2 09/12] x86/msr: Use the alternatives mechanism for WRMSR

["Jürgen Groß" <jgross@suse.com>]

[-- Attachment #1.1.1: Type: text/plain, Size: 1886 bytes --]

On 01.10.25 12:50, Peter Zijlstra wrote:
> On Wed, Oct 01, 2025 at 10:49:31AM +0200, Juergen Gross wrote:
> 
>> Thinking more about that I believe there are additional problems:
>>
>> Having overlapping alternatives not starting at the same address will result
>> in problems with length padding of the outer alternative in case the inner
>> one starting later is extending past the end of the outer one. This might be
>> possible to handle, but it will be tedious.
> 
> Yes, this must not happen.
> 
>> Using your idea with pv_ops could result in the inner alternative not being
>> at the start of the outer alternative AND being not the initial code. This
>> would result in patching in the .altinstructions area instead of the normal
>> .text site, resulting in possible loss of a patching action if the patched
>> area would have been used as a replacement before.
> 
> Not quite, the nested alternative was in the orig_insn part. So it would
> result in patching the orig text twice, once from the inner (which comes
> first in the patch list) and then once again from the outer (which comes
> later).

Yes, but that was the native case only.

With pv_ops this would mean the original instruction would be the
paravirt-call, resulting in your construct becoming an inner nesting level.

> 
>> So in my opinion allowing alternatives to nest without all inner levels
>> starting at the same location as the outermost level would be a receipt for
>> failure.
> 
> Certainly tricky, no argument there.
> 
>> I think I'll write another patch to BUG() in case such a situation is being
>> detected.
> 
> Fair enough; we should not currently have any such cases. And going by
> my attempt to make it work, its going to be really tricky in any case.
> 
> But please put on a comment on why, which explains the constraints.

Agreed.


Juergen

[-- Attachment #1.1.2: OpenPGP public key --]
[-- Type: application/pgp-keys, Size: 3743 bytes --]

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