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X-CSE-ConnectionGUID: jTIqakz/Qq+7r3c9Ziqcpw== X-CSE-MsgGUID: UzEWM1IISaO64Aod1lbWpA== X-IronPort-AV: E=McAfee;i="6800,10657,11719"; a="85048042" X-IronPort-AV: E=Sophos;i="6.21,324,1763452800"; d="scan'208";a="85048042" Received: from orviesa001.jf.intel.com ([10.64.159.141]) by fmvoesa104.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 04 Mar 2026 07:25:50 -0800 X-CSE-ConnectionGUID: vtF7OjPBQoG3vn+8rgx1LQ== X-CSE-MsgGUID: rvSzvX3eRUmvBcV4MnJciA== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.21,324,1763452800"; d="scan'208";a="256253427" Received: from aschende-mobl.amr.corp.intel.com (HELO [10.125.108.205]) ([10.125.108.205]) by smtpauth.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 04 Mar 2026 07:25:47 -0800 Message-ID: <7ab8d3af-b4f5-481c-ab2e-059ddd7e718e@intel.com> Date: Wed, 4 Mar 2026 07:25:53 -0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 3/7] x86/sev: add support for RMPOPT instruction To: Sean Christopherson , Ashish Kalra Cc: tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, peterz@infradead.org, thomas.lendacky@amd.com, herbert@gondor.apana.org.au, davem@davemloft.net, ardb@kernel.org, pbonzini@redhat.com, aik@amd.com, Michael.Roth@amd.com, KPrateek.Nayak@amd.com, Tycho.Andersen@amd.com, Nathan.Fontenot@amd.com, jackyli@google.com, pgonda@google.com, rientjes@google.com, jacobhxu@google.com, xin@zytor.com, pawan.kumar.gupta@linux.intel.com, babu.moger@amd.com, dyoung@redhat.com, nikunj@amd.com, john.allen@amd.com, darwi@linutronix.de, linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org, kvm@vger.kernel.org, linux-coco@lists.linux.dev References: <8dc0198f1261f5ae4b16388fc1ffad5ddb3895f9.1772486459.git.ashish.kalra@amd.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 3/4/26 07:01, Sean Christopherson wrote: > I don't see any performance data in either posted version. Bluntly, this series > isn't going anywhere without data to guide us. E.g. comments like this from v1 > > : And there is a cost associated with re-enabling the optimizations for all > : system RAM (even though it runs as a background kernel thread executing RMPOPT > : on different 1GB regions in parallel and with inline cond_resched()'s), > : we don't want to run this periodically. > > suggest there is meaningful cost associated with the scan. Well the RMP is 0.4% of the size of system memory, and I assume that you need to scan the whole table. There are surely shortcuts for 2M pages, but with 4k, that's ~8.5GB of RMP table for 2TB of memory. That's an awful lot of memory traffic for each CPU. It'll be annoying to keep a refcount per 1GB of paddr space. One other way to do it would be to loosely mirror the RMPOPT bitmap and keep our own bitmap of 1GB regions that _need_ RMPOPT run on them. Any private=>shared conversion sets a bit in the bitmap and schedules some work out in the future. It could also be less granular than that. Instead of any private=>shared conversion, the RMPOPT scan could be triggered on VM destruction which is much more likely to result in RMPOPT doing anything useful. BTW, I assume that the RMPOPT disable machinery is driven from the INVLPGB-like TLB invalidations that are a part of the SNP shared=>private conversions. It's a darn shame that RMPOPT wasn't broadcast in the same way. It would save the poor OS a lot of work. The RMPOPT table is per-cpu of course, but I'm not sure what keeps *a* CPU from broadcasting its success finding an SNP-free physical region to other CPUs. tl;dr: I agree with you. The cost of these scans is going to be annoying, and it's going to need OS help to optimize it.