Re: [PATCH 5/6] x86/paravirt: Switch MSR access pv_ops functions to instruction interfaces

["Jürgen Groß" <jgross@suse.com>]

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On 13.06.25 09:31, Xin Li wrote:
> On 6/11/2025 5:58 AM, Juergen Gross wrote:
>>> Here is a patch I cooked.  I added an ALTERNATIVE() hack because the new 
>>> instructions can't be more than 6 bytes long.  But with the patch you
>>> just sent, it shouldn't be needed.
>>
>> I have meanwhile dropped the patch copying the original indirect call.
>>
>> Reason is that I'm seeing a potential risk with current alternative
>> patching when using ALTERNATIVE_[23](): depending on the tested features
>> it might happen that an instruction sequence not suitable for the current
>> runtime environment is patched in as an intermediate step. In case there
>> is an interrupt happening just then AND the handling of the interrupt is
>> using the patch site, this could result in crashes or undefined behavior.
> 
> Oh, I had assumed that Linux disables interrupts during the patching
> process. Just out of curiosity, why are interrupts allowed in this case?

Interrupts are disabled within text_poke_early() while patching a single
instance.

I guess keeping interrupts disabled during the complete apply_alternatives()
handling would potentially result in a too long period without handling any
interrupts.


Juergen

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