Re: [PATCH 0/4] x86: Drop cross-vendor support

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 23/01/2026 3:45 pm, Alejandro Vallejo wrote:
> On Fri Jan 23, 2026 at 3:05 PM CET, Jan Beulich wrote:
>> On 23.01.2026 13:10, Alejandro Vallejo wrote:
>>> On Thu Jan 22, 2026 at 7:16 PM CET, Teddy Astie wrote:
>>>> Le 22/01/2026 à 18:44, Alejandro Vallejo a écrit :
>>>>> On Thu Jan 22, 2026 at 6:10 PM CET, Andrew Cooper wrote:
>>>>>> On 22/01/2026 4:49 pm, Alejandro Vallejo wrote:
>>>>>>> Open question unrelated to the series: Does it make sense to 
>>>>>>> conditionalise the
>>>>>>> MSR handlers for non intercepted MSRs on HVM_FEP?
>>>>>> I'm not quite sure what you're asking here.
>>>>>>
>>>>>> ~Andrew
>>>>> The handlers for LSTAR and the like are dead code with !CONFIG_HVM_FEP as 
>>>>> far
>>>>> as I can tell. The question I'm asking is whether there is another code 
>>>>> path
>>>>> that might invoke MSR handlers for non-intercepted MSRs. I can't see it, 
>>>>> but
>>>>> I'm not sure.
>>>>>
>>>>> If there isn't I'm considering (conditionally) getting rid of them.
>>>>>
>>>> I think you can enter this path by making the guest execute directly or 
>>>> indirectly a rdmsr in a emulated path (there are some cases like certain 
>>>> cases of real mode or maybe vm introspection). I don't think that FEP is 
>>>> the only way to do that.
>>> For the emulation path, I think HVM_FEP is the only means to trigger it, as
>>> neither {rd,wr}msr access memory. VMI (as you mention) and nSVM (as Andrew 
>>> did)
>>> do make sense, but I don't see any others. I don't see how real mode could 
>>> cause
>>> anything (I'm fuzzy on VMX, but I _think_ instructions do execute, just in
>>> a weird paging-on mode akin to v8086).
>> Iirc there's still the situation where for PAE shadow code tries to emulate 
>> up
>> to 4 insns in a row, in the hope to find the other half of a full PTE update.
>>
>> Jan
> That's a rather obscure optimisation, thanks for bringing it up.
> multi.c:sh_page_fault() is rather... opaque to just look at it and expect to
> find anything.

Shadow's hvm_shadow_emulator_ops doesn't fill in the MSR hooks, so
should at least abort when encountering this case.

But there are a lot of instructions which can fit in the restriction to
simple read/write/cmpxchg.

~Andrew