LLVM Merges Support For AMD's RMPCHKD, RMPREAD & RMPOPT Instructions

With the Linux kernel still working toward landing support for AMD's RMPOPT optimization for helping to reduce overhead on SEV-SNP enabled servers, the LLVM/Clang compiler has in turn prepped for the milestone by landing support for the RMPOPT, RMPREAD, and RMPCHKD instructions for Zen 5 and newer. While initially it was assumed RMPOPT was going to be a feature of Zen 6 given the timing, RMPOPT is supported on Zen 5 EPYC processors already too and this software enablement just turned out to be rather belated. Sent out last week was the v13 patches for enabling RMPOPT. We'll see if this code is deemed ready by the time of the Linux v7.4 merge window in late October. As a reminder, RMPOPT is a new instruction to help minimize the performance overhead of Reverse Map Table (RMP) checks on the hypervisor and for non-SNP guests by letting RMP checks be skipped in certain scenarios. With RMPOPT these checks can be skipped when 1G regions of memory are known not to contain any SEV-SNP guest memory. Along with RMPOPT is RMPREAD as the instruction for reading Reverse Map Table entries. RMPCHKD is used for checking the dirty status bit on a range of guest private pages in the Reverse Map Table. Merged overnight to the LLVM toolchain Git code was this pull to "support further Zen5 and Zen6 instructions." Those additional instructions are just the RMPREAD, RMPOPT, and RMPCHKD. It's not clear why it's taken so long for AMD to enable the RMPOPT support with it being found on 5th Gen EPYC processors the past two years but at least it appears to be approaching the finish line now and just in time for 6th Gen EPYC "Venice" servers debuting and continuing with this SEV-SNP confidential VM optimization.