Re: [PATCH v3 4/6] x86/sev: Add interface to re-enable RMP optimizations.

[Kalra, Ashish]

On 3/30/2026 6:33 PM, Dave Hansen wrote:
> The subject seems rather imprecise. This both adds a function to
> "re-enable RMP optimizations" *AND* calls it.
> 
>> RMPOPT table is a per-processor table which indicates if 1GB regions of
>> physical memory are entirely hypervisor-owned or not.
> 
> It's per-core, right? Why not just be precise about it?
>

Ok.
 
>> When performing host memory accesses in hypervisor mode as well as
>> non-SNP guest mode, the processor may consult the RMPOPT table to
>> potentially skip an RMP access and improve performance.
>>
>> Events such as RMPUPDATE or SNP_INIT can clear RMP optimizations. Add
>> an interface to re-enable those optimizations.
> 
> 
>> +int snp_perform_rmp_optimization(void)
>> +{
>> +	if (!cpu_feature_enabled(X86_FEATURE_RMPOPT))
>> +		return -EINVAL;
>> +
>> +	if (!cc_platform_has(CC_ATTR_HOST_SEV_SNP))
>> +		return -EINVAL;
>> +
>> +	if (!(rmp_cfg & MSR_AMD64_SEG_RMP_ENABLED))
>> +		return -EINVAL;
> 
> This seems wrong. How about we just make 'X86_FEATURE_RMPOPT' the one
> true source of RMP support?
>

 
Ok, i will work on that.

> If you don't have CC_ATTR_HOST_SEV_SNP you:
> 
> 	setup_clear_cpu_cap(X86_FEATURE_RMPOPT)
> 
> Ditto for MSR_AMD64_SEG_RMP_ENABLED.
> 
> It could also potentially replace the 'rmpopt_wq' checks.
> 
>> +	rmpopt_all_physmem(FALSE);
>> +
>> +	return 0;
>> +}
>> +EXPORT_SYMBOL_GPL(snp_perform_rmp_optimization);
>> +
>>  void __snp_leak_pages(u64 pfn, unsigned int npages, bool dump_rmp)
>>  {
>>  	struct page *page = pfn_to_page(pfn);
>> diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
>> index aebf4dad545e..0cbe828d204c 100644
>> --- a/drivers/crypto/ccp/sev-dev.c
>> +++ b/drivers/crypto/ccp/sev-dev.c
>> @@ -1476,6 +1476,10 @@ static int __sev_snp_init_locked(int *error, unsigned int max_snp_asid)
>>  	}
>>  
>>  	snp_hv_fixed_pages_state_update(sev, HV_FIXED);
>> +
>> +	/* SNP_INIT clears the RMPOPT table, re-enable RMP optimizations */
>> +	snp_perform_rmp_optimization();
> 
> Ahhh, so this isn't happening at boot, it happens when kvm_amd.ko gets
> loaded? That escaped me until now. It would be nice to mention
> somewhere, please.

Well, as of now, it is happening at both boot time and here after SNP_INIT_EX,
as SNP_INIT clears the RMPOPT table contents to 0, but eventually, when SNP enable
is moved completely out of snp_rmptable_init() and only done when kvm_amd.ko
gets loaded, then it will only happen here.

Eventually, call to "configure_and_enable_rmpopt()" (setup_rmpopt()) will move out
of snp_rmptable_init() and get called from here.

> 
> There is basically no naming difference between
> snp_perform_rmp_optimization() and rmpopt_all_physmem(). Can you just
> get this all down to a single function, please?
> 
> If you really have a reason to have a scan now and scan later mode, just
> do this:
> 
> 	rmpopt_all_physmem(RMPOPT_SCAN_NOW);
> 
> and:
> 
> 	rmpopt_all_physmem(RMPOPT_SCAN_LATER);
> 
> *That* function can do the X86_FEATURE_RMPOPT check.

The only thing is that this is an exported function out of the x86 platform
code, so will probably need to be renamed as snp_rmpopt_all_physmem().

Thanks,
Ashish