Re: [PATCH v3 1/1] x86/mce/amd: Guard SMCA DESTAT access on non-SMCA machines

[William Roche]

I just wanted to give a small precision about the VM error relay mechanism:

On 3/17/26 19:17, Borislav Petkov wrote:
> Rewrote it again after talking to Yazen. A patch needs to have the proper
> justification why it exists!
>
> x86/mce/amd: Check SMCA feature bit before accessing SMCA MSRs
>
> People do effort to inject MCEs into guests in order to simulate/test
> handling of hardware errors. The real use case behind it is testing the
> handling of SIGBUS which the memory failure code sends to the process.
>   
> If that process is QEMU, instead of killing the whole guest, the MCE can
> be injected into the guest kernel so that latter can attempt proper
> handling and kill the user *process*  in the guest, instead, which
> caused the MCE. The assumption being here that the whole injection flow
> can supply enough information that the guest kernel can poinpoint the
> right process. But that's a different topic...


Relaying the error to the guest doesn't only have a value to target a VM 
process but also deal with free memory or clean file cache memory 
impacted etc... Cases where a memory error may not crash the kernel can 
benefit to the VM too (Kernel RAS features that are, as you said, a 
different topic :) )

There is also a small typo in "pinpoint"

>   
> Regardless of virtualization or not, access to SMCA-specific registers
> like MCA_DESTAT should only be done after having checked the smca
> feature bit. And there are AMD machines like Bulldozer (the one before
> Zen1) which do support deferred errors but are not SMCA machines.
>   
> Therefore, properly check the feature bit before accessing related MSRs.
>   
>    [ bp: Rewrite commit message. ]


Thank you very much for your feedback !
William.