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([2001:b07:6468:f312:5e2c:eb9a:a8b6:fd3e]) by smtp.gmail.com with ESMTPSA id u17sm7779485edt.67.2021.07.06.14.58.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 06 Jul 2021 14:58:11 -0700 (PDT) To: Eduardo Habkost Cc: Pawan Gupta , Thomas Gleixner , Borislav Petkov , Jonathan Corbet , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , x86@kernel.org, "H. Peter Anvin" , "Paul E. McKenney" , Randy Dunlap , Andrew Morton , "Maciej W. Rozycki" , Viresh Kumar , Vlastimil Babka , Tony Luck , Sean Christopherson , Kyung Min Park , Fenghua Yu , Ricardo Neri , Tom Lendacky , Juergen Gross , Krish Sadhukhan , Kan Liang , Joerg Roedel , Victor Ding , Srinivas Pandruvada , Brijesh Singh , Dave Hansen , Mike Rapoport , Anthony Steinhauser , Anand K Mistry , Andi Kleen , Miguel Ojeda , Nick Desaulniers , Joe Perches , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, kvm@vger.kernel.org References: <20210706195233.h6w4cm73oktfqpgz@habkost.net> <4cc2c5fe-2153-05c5-dedd-8cb650753740@redhat.com> From: Paolo Bonzini Subject: Re: [PATCH 4/4] x86/tsx: Add cmdline tsx=fake to not clear CPUID bits RTM and HLE Message-ID: <671be35f-220a-f583-aa31-3a2da7dae93a@redhat.com> Date: Tue, 6 Jul 2021 23:58:08 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-perf-users@vger.kernel.org On 06/07/21 23:33, Eduardo Habkost wrote: > On Tue, Jul 6, 2021 at 5:05 PM Paolo Bonzini wrote: >> It's a bit tricky, because HLE and RTM won't really behave well. An old >> guest that sees RTM=1 might end up retrying and aborting transactions >> too much. So I'm not sure that a QEMU "-cpu host" guest should have HLE >> and RTM enabled. > > Is the purpose of GET_SUPPORTED_CPUID to return what is supported by > KVM, or to return what "-cpu host" should enable by default? They are > conflicting requirements in this case. In theory there is GET_EMULATED_CPUID for the former, so it should be the latter. In practice neither QEMU nor Libvirt use it; maybe now we have a good reason to add it, but note that userspace could also check host RTM_ALWAYS_ABORT. > Returning HLE=1,RTM=1 in GET_SUPPORTED_CPUID makes existing userspace > take bad decisions until it's updated. > > Returning HLE=0,RTM=0 in GET_SUPPORTED_CPUID prevents existing > userspace from resuming existing VMs (despite being technically > possible). > > The first option has an easy workaround that doesn't require a > software update (disabling HLE/RTM in the VM configuration). The > second option doesn't have a workaround. I'm inclined towards the > first option. The default has already been tsx=off for a while though, so checking either GET_EMULATED_CPUID or host RTM_ALWAYS_ABORT in userspace might also be feasible for those that are still on tsx=on. Paolo