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Re: [PATCH 0/4] x86: Drop cross-vendor support
- To: Teddy Astie <teddy.astie@xxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
- Date: Fri, 23 Jan 2026 13:10:13 +0100
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- Cc: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>, Community Manager <community.manager@xxxxxxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Jason Andryuk <jason.andryuk@xxxxxxx>
- Delivery-date: Fri, 23 Jan 2026 12:10:37 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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).
I'll munch on the idea I bit longer. If I can't come up with any other cases
I'll send something to remove that dead code for the cases in which it's truly
dead.
Cheers,
Alejandro
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