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h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=KPEIW0mOtLf4M5A0uDTpd1I6Chv53XOVonBfR2smAgqkJQjn2Wj4l0rJn2JywBej5 YXwtshhuGkybf4Vb/K9/y5cmYLzUR1twsJUh+ZLmZSJVO9LtiMlq0YnJ0q9IDapEj+ 1dYrsljFe08yXhnHcxijgTZIJScll8+azU4en7BPWvHRV9iJCemunLgpHiKXOMAF88 Q5UBQk1oQ2lckzxXG2I8q+tpcQEJG9sNjHGIY4jlWVKfWpdJLB0DSPTkA64CftqxxU 8CEeKXGkGTsVBSe5rZs1OTD2FYyapuE27QnH65SfXYWTgdm15c5LNhuNJo1mdzqouW cx09OumVwz0lw== Message-ID: <2dc165c7-e3fd-48f6-bcfb-c2119fc94a54@zytor.com> Date: Wed, 25 Jun 2025 11:05:04 -0700 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 v4 15/19] KVM: x86: Allow FRED/LKGS to be advertised to guests To: Sean Christopherson Cc: pbonzini@redhat.com, kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, corbet@lwn.net, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, andrew.cooper3@citrix.com, luto@kernel.org, peterz@infradead.org, chao.gao@intel.com, xin3.li@intel.com References: <20250328171205.2029296-1-xin@zytor.com> <20250328171205.2029296-16-xin@zytor.com> Content-Language: en-US From: Xin Li Autocrypt: addr=xin@zytor.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 6/24/2025 9:38 AM, Sean Christopherson wrote: > The shortlog (and changelog intro) are wrong. KVM isn't allowing FRED/LKGS to > be advertised to the guest. Userspace can advertise whatever it wants. The guest > will break badly without KVM support, but that doesn't stop userspace from > advertising a bogus vCPU model. > > KVM: x86: Advertise support for FRED/LKGS to userspace > > On Fri, Mar 28, 2025, Xin Li (Intel) wrote: >> From: Xin Li >> >> Allow FRED/LKGS to be advertised to guests after changes required to > > Please explain what LKGS is early in the changelog. I assumed it was a feature > of sorts; turns out it's a new instruction. > > Actually, why wait this long to enumerate support for LKGS? I.e. why not have a > patch at the head of the series to enumerate support for LKGS? IIUC, LKGS doesn't > depend on FRED. I will send LKGS as a separate patch, thus if you prefer you can take it before the KVM FRED patch set. > >> enable FRED in a KVM guest are in place. >> >> LKGS is introduced with FRED to completely eliminate the need to swapgs >> explicilty, because >> >> 1) FRED transitions ensure that an operating system can always operate >> with its own GS base address. >> >> 2) LKGS behaves like the MOV to GS instruction except that it loads >> the base address into the IA32_KERNEL_GS_BASE MSR instead of the >> GS segment’s descriptor cache, which is exactly what Linux kernel >> does to load a user level GS base. Thus there is no need to SWAPGS >> away from the kernel GS base and an execution of SWAPGS causes #UD >> if FRED transitions are enabled. >> >> A FRED CPU must enumerate LKGS. When LKGS is not available, FRED must >> not be enabled. >> >> Signed-off-by: Xin Li >> Signed-off-by: Xin Li (Intel) >> Tested-by: Shan Kang >> --- >> arch/x86/kvm/cpuid.c | 2 ++ >> 1 file changed, 2 insertions(+) >> >> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c >> index 5e4d4934c0d3..8f290273aee1 100644 >> --- a/arch/x86/kvm/cpuid.c >> +++ b/arch/x86/kvm/cpuid.c >> @@ -992,6 +992,8 @@ void kvm_set_cpu_caps(void) >> F(FZRM), >> F(FSRS), >> F(FSRC), >> + F(FRED), >> + F(LKGS), > > These need to be X86_64_F, no? Yes. Both LKGS and FRED are 64-bit only features. However I assume KVM is 64-bit only now, so X86_64_F is essentially F, right? Thanks! Xin