Re: [PATCH 5/6] x86/sev: Use configfs to re-enable RMP optimizations.

[Kalra, Ashish]

Hello Dave, 

On 2/17/2026 4:19 PM, Dave Hansen wrote:
> On 2/17/26 12:11, Ashish Kalra wrote:
>> From: Ashish Kalra <ashish.kalra@xxxxxxx>
>>
>> Use configfs as an interface to re-enable RMP optimizations at runtime
>>
>> When SNP guests are launched, RMPUPDATE disables the corresponding
>> RMPOPT optimizations. Therefore, an interface is required to manually
>> re-enable RMP optimizations, as no mechanism currently exists to do so
>> during SNP guest cleanup.
> 
> Is this like a proof-of-concept to poke the hardware and show it works?
> Or, is this intended to be the way that folks actually interact with
> SEV-SNP optimization in real production scenarios?
> 
> Shouldn't freeing SEV-SNP memory back to the system do this
> automatically? Worst case, keep a 1-bit-per-GB bitmap of memory that's
> been freed and schedule_work() to run in 1 or 10 or 100 seconds. That
> should batch things up nicely enough. No?

Actually, the RMPOPT implementation is going to be a multi-phased development.

In the first phase (which is this patch-series) we enable RMPOPT globally, and let RMPUPDATE(s)
slowly switch it off over time as SNP guest spin up, and then in phase#2 once 1GB hugetlb is in place,
we enable re-issuing of RMPOPT during 1GB page cleanup.

So automatic re-issuing of RMPOPT will be done when SNP guests are shutdown and as part of 
SNP guest cleanup once 1GB hugetlb support (for guest_memfd) has been merged. 

As currently, i.e, as part of this patch series, there is no mechanism to re-issue RMPOPT
automatically as part of SNP guest cleanup, therefore this support exists to doing it
manually at runtime via configfs.

I will describe this multi-phased RMPOPT implementation plan in the cover letter for 
next revision of this patch series.

Thanks,
Ashish

> 
> I can't fathom that users don't want this to be done automatically for them.
> 
> Is the optimization scan really expensive or something? 1GB of memory
> should have a small number of megabytes of metadata to scan.