Re: [PATCH v4 0/7] Add RMPOPT support.

[Kalra, Ashish]

Hello Dave, Sean,

Looking forward to your feedback, comments, thoughts on RMPOPT v4 patch series.

Thanks,
Ashish

On 4/13/2026 2:42 PM, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.kalra@xxxxxxx>
> 
> In the SEV-SNP architecture, hypervisor and non-SNP guests are subject
> to RMP checks on writes to provide integrity of SEV-SNP guest memory.
> 
> The RMPOPT architecture enables optimizations whereby the RMP checks
> can be skipped if 1GB regions of memory are known to not contain any
> SNP guest memory.
> 
> RMPOPT is a new instruction designed to minimize the performance
> overhead of RMP checks for the hypervisor and non-SNP guests.
> 
> RMPOPT instruction currently supports two functions. In case of the
> verify and report status function the CPU will read the RMP contents,
> verify the entire 1GB region starting at the provided SPA is HV-owned.
> For the entire 1GB region it checks that all RMP entries in this region
> are HV-owned (i.e, not in assigned state) and then accordingly updates
> the RMPOPT table to indicate if optimization has been enabled and
> provide indication to software if the optimization was successful.
> 
> In case of report status function, the CPU returns the optimization
> status for the 1GB region.
> 
> The RMPOPT table is managed by a combination of software and hardware.
> Software uses the RMPOPT instruction to set bits in the table,
> indicating that regions of memory are entirely HV-owned.  Hardware
> automatically clears bits in the RMPOPT table when RMP contents are
> changed during RMPUPDATE instruction.
> 
> For more information on the RMPOPT instruction, see the AMD64 RMPOPT
> technical documentation.
> 
> As SNP is enabled by default the hypervisor and non-SNP guests are
> subject to RMP write checks to provide integrity of SNP guest memory.
> 
> This patch-series adds support to enable RMP optimizations for up to
> 2TB of system RAM across the system and allow RMPUPDATE to disable
> those optimizations as SNP guests are launched.
> 
> Support for RAM larger than 2 TB will be added in follow-on series.
> 
> This series also introduces support to re-enable RMP optimizations
> during SNP guest termination, after guest pages have been converted
> back to shared.
> 
> RMP optimizations are performed asynchronously by queuing work on a
> dedicated workqueue after a 10 second delay.
> 
> Delaying work allows batching of multiple SNP guest terminations.
> 
> Once 1GB hugetlb guest_memfd support is merged, support for
> re-enabling RMPOPT optimizations during 1GB page cleanup will be added
> in follow-on series.
> 
> Additionally add debugfs interface to report per-CPU RMPOPT status
> across all system RAM.
> 
> v4:
> - Add new wrmsrq_on_cpus() helper to write same u64 value to a
>   per-CPU MSR across a cpumask without per-cpu struct allocation
>   overhead. 
> - Rename configure_and_enable_rmpopt() to snp_setup_rmpopt().
> - Use wrmsrq_on_cpus() instead of wrmsrq_on_cpu() loop for
>   programming RMPOPT_BASE MSRs.
> - Add setup_clear_cpu_cap(X86_FEATURE_RMPOPT) if segmented RMP
>   setup fails or workqueue allocation fails.
> - Add X86_FEATURE_RMPOPT feature clear logic in amd_cc_platform_clear()
>   for CC_ATTR_HOST_SEV_SNP.
> - All of the above allow checking for only X86_FEATURE_RMPOPT for both
>   RMPOPT setup/enable and RMP re-optimizations.
> - Rename snp_perform_rmp_optimization() to snp_rmpopt_all_physmem().
> - Split rmpopt() into rmpopt() and rmpopt_smp() for SMP callback use.
> - Introduce separate rmpopt_report_cpumask for debugfs reporting,
>   distinct from rmpopt_cpumask used for primary thread tracking.
> - Remove snp_perform_rmp_optimization() call from __sev_snp_init_locked() 
>   and instead setup and enable RMPOPT after SNP is enabled and 
>   initialized.
> 
> v3:
> - Drop all RMPOPT kthread support and introduce adding custom and
>   dedicated workqueue to schedule delayed and asynchronous RMPOPT work.
> - Drop the guest_memfd inode cleanup interface and add support to
>   re-enable RMP optimizations during guest shutdown using the
>   asynchronous and delayed workqueue interface.
> - Introduce new __rmpopt() helper and rmpopt() and
>   rmpopt_report_status() wrappers on top which use rax and rcx
>   parameters to closely match RMPOPT specs.
> - Use new optimized RMPOPT loop to issue RMPOPT instructions on all
>   system RAM upto 2TB and all CPUs, by optimizing each range on one CPU
>   first, then let other CPUs execute RMPOPT in parallel so they can skip
>   most work as the range has already been optimized.
> - Also add support for running the optimized RMPOPT loop only on
>   one thread per core.
> - Replace all PUD_SIZE references with SZ_1G to conform to 1GB regions
>   as specified by RMPOPT specifications and not be dependent on PUD_SIZE
>   which makes the RMPOPT patch-set independent of x86 page table sizes.
> - Use wrmsrq_on_cpu() to program the RMPOPT_BASE MSR registers on
>   all CPUs that removes all ugly casting to use on_each_cpu_mask().
> - Fix inline commits and patch commit messages
> 
> 
> v2:
> - Drop all NUMA and Socket configuration and enablement support and
>   enable RMPOPT support for up to 2TB of system RAM.
> - Drop get_cpumask_of_primary_threads() and enable per-core RMPOPT
>   base MSRs and issue RMPOPT instruction on all CPUs.
> - Drop the configfs interface to manually re-enable RMP optimizations.
> - Add new guest_memfd cleanup interface to automatically re-enable
>   RMP optimizations during guest shutdown.
> - Include references to the public RMPOPT documentation.
> - Move debugfs directory for RMPOPT under architecuture specific
>   parent directory.
> 
> Ashish Kalra (7):
>   x86/cpufeatures: Add X86_FEATURE_AMD_RMPOPT feature flag
>   x86/msr: add wrmsrq_on_cpus helper
>   x86/sev: Initialize RMPOPT configuration MSRs
>   x86/sev: Add support to perform RMP optimizations asynchronously
>   x86/sev: Add interface to re-enable RMP optimizations.
>   KVM: SEV: Perform RMP optimizations on SNP guest shutdown
>   x86/sev: Add debugfs support for RMPOPT
> 
>  arch/x86/coco/core.c               |   1 +
>  arch/x86/include/asm/cpufeatures.h |   2 +-
>  arch/x86/include/asm/msr-index.h   |   3 +
>  arch/x86/include/asm/msr.h         |   5 +
>  arch/x86/include/asm/sev.h         |   4 +
>  arch/x86/kernel/cpu/scattered.c    |   1 +
>  arch/x86/kvm/svm/sev.c             |   2 +
>  arch/x86/lib/msr-smp.c             |  20 +++
>  arch/x86/virt/svm/sev.c            | 271 ++++++++++++++++++++++++++++-
>  drivers/crypto/ccp/sev-dev.c       |   3 +
>  10 files changed, 310 insertions(+), 2 deletions(-)
> 
> --
> 2.43.0
> 
>