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AJvYcCWFmKnfu51ZfQSXcOk8kVTG5BQHmDTBDrygydDRRgdJGh1a3jQgYfga/GtWif+0D9VdUtu0Gd6iEklZ/Y3wiQzuQNo8MGuxFLCyCHZ6 X-Gm-Message-State: AOJu0YzHPWP4pvE+WVbhypd9J57mYsX6T/CxXCtKEKEfE9gy+oZ8PoRU CMljH9EQ2WpYFc04HyfWgk++JR0Ww223NWZZ+x7zWM/cAJ9fITqGV+Uc4lZPXWg31Wjt9FWJtrX VhPq/z4vaSVmwwTo+8+Ie7pdHcAOmee3xfzv5 X-Google-Smtp-Source: AGHT+IGvihwbUhusHWWPdSiPPgnic5+W6UFEhcbEi6LwWX3TzgMS2Jus/PkdwrB0oVy3pMw/VwbpYLaEqgYy9fp4c1M= X-Received: by 2002:a50:f68d:0:b0:57c:bb0d:5e48 with SMTP id 4fb4d7f45d1cf-5872f673e4bmr22935a12.2.1719600498008; Fri, 28 Jun 2024 11:48:18 -0700 (PDT) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 References: <20240626073719.5246-1-amit@kernel.org> In-Reply-To: From: Jim Mattson Date: Fri, 28 Jun 2024 11:48:01 -0700 Message-ID: Subject: Re: [PATCH v2] KVM: SVM: let alternatives handle the cases when RSB filling is required To: Sean Christopherson Cc: Amit Shah , pbonzini@redhat.com, x86@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, amit.shah@amd.com, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, kim.phillips@amd.com, david.kaplan@amd.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, Jun 28, 2024 at 9:09=E2=80=AFAM Sean Christopherson wrote: > > On Wed, Jun 26, 2024, Amit Shah wrote: > > --- > > arch/x86/kvm/svm/vmenter.S | 8 ++------ > > 1 file changed, 2 insertions(+), 6 deletions(-) > > > > diff --git a/arch/x86/kvm/svm/vmenter.S b/arch/x86/kvm/svm/vmenter.S > > index a0c8eb37d3e1..2ed80aea3bb1 100644 > > --- a/arch/x86/kvm/svm/vmenter.S > > +++ b/arch/x86/kvm/svm/vmenter.S > > @@ -209,10 +209,8 @@ SYM_FUNC_START(__svm_vcpu_run) > > 7: vmload %_ASM_AX > > 8: > > > > -#ifdef CONFIG_MITIGATION_RETPOLINE > > /* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET= ! */ > > - FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLI= NE > > -#endif > > + FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_VME= XIT > > Out of an abundance of paranoia, shouldn't this be? > > FILL_RETURN_BUFFER %_ASM_CX, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_VME= XIT,\ > X86_FEATURE_RSB_VMEXIT_LITE > > Hmm, but it looks like that would incorrectly trigger the "lite" flavor f= or > families 0xf - 0x12. I assume those old CPUs aren't affected by whatever= on earth > EIBRS_PBRSB is. > > /* AMD Family 0xf - 0x12 */ > VULNWL_AMD(0x0f, NO_MELTDOWN | NO_SSB | NO_L1TF | NO_MDS |= NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO | NO_BHI), > VULNWL_AMD(0x10, NO_MELTDOWN | NO_SSB | NO_L1TF | NO_MDS |= NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO | NO_BHI), > VULNWL_AMD(0x11, NO_MELTDOWN | NO_SSB | NO_L1TF | NO_MDS |= NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO | NO_BHI), > VULNWL_AMD(0x12, NO_MELTDOWN | NO_SSB | NO_L1TF | NO_MDS |= NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO | NO_BHI), > > /* FAMILY_ANY must be last, otherwise 0x0f - 0x12 matches won't w= ork */ > VULNWL_AMD(X86_FAMILY_ANY, NO_MELTDOWN | NO_L1TF | NO_MDS | = NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO | NO_EIBRS_PBRSB | NO_BHI), > VULNWL_HYGON(X86_FAMILY_ANY, NO_MELTDOWN | NO_L1TF | NO_MDS | = NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO | NO_EIBRS_PBRSB | NO_BHI), Your assumption is correct. As for why the cpu_vuln_whitelist[] doesn't say so explicitly, you need to read between the lines... > /* > * AMD's AutoIBRS is equivalent to Intel's eIBRS - use the Intel f= eature > * flag and protect from vendor-specific bugs via the whitelist. > * > * Don't use AutoIBRS when SNP is enabled because it degrades host > * userspace indirect branch performance. > */ > if ((x86_arch_cap_msr & ARCH_CAP_IBRS_ALL) || > (cpu_has(c, X86_FEATURE_AUTOIBRS) && > !cpu_feature_enabled(X86_FEATURE_SEV_SNP))) { > setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED); > if (!cpu_matches(cpu_vuln_whitelist, NO_EIBRS_PBRSB) && > !(x86_arch_cap_msr & ARCH_CAP_PBRSB_NO)) > setup_force_cpu_bug(X86_BUG_EIBRS_PBRSB); > } Families 0FH through 12H don't have EIBRS or AutoIBRS, so there's no cpu_vuln_whitelist[] lookup. Hence, no need to set the NO_EIBRS_PBRSB bit, even if it is accurate. > > > > /* Clobbers RAX, RCX, RDX. */ > > RESTORE_HOST_SPEC_CTRL > > @@ -348,10 +346,8 @@ SYM_FUNC_START(__svm_sev_es_vcpu_run) > > > > 2: cli > > > > -#ifdef CONFIG_MITIGATION_RETPOLINE > > /* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET= ! */ > > - FILL_RETURN_BUFFER %rax, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLINE > > -#endif > > + FILL_RETURN_BUFFER %rax, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_VMEXIT > > > > /* Clobbers RAX, RCX, RDX, consumes RDI (@svm) and RSI (@spec_ctr= l_intercepted). */ > > RESTORE_HOST_SPEC_CTRL > > -- > > 2.45.2 > > >