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[46.102.197.194]) by smtp.gmail.com with ESMTPSA id cz6-20020a056214088600b00696b282f582sm720350qvb.97.2024.04.11.02.38.11 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 11 Apr 2024 02:38:13 -0700 (PDT) Message-ID: Date: Thu, 11 Apr 2024 10:38:10 +0100 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] KVM: x86: Set BHI_NO in guest when host is not affected by BHI To: Alexandre Chartre , x86@kernel.org, kvm@vger.kernel.org Cc: linux-kernel@vger.kernel.org, daniel.sneddon@linux.intel.com, pawan.kumar.gupta@linux.intel.com, tglx@linutronix.de, konrad.wilk@oracle.com, peterz@infradead.org, gregkh@linuxfoundation.org, seanjc@google.com, dave.hansen@linux.intel.com, nik.borisov@suse.com, kpsingh@kernel.org, longman@redhat.com, bp@alien8.de, pbonzini@redhat.com References: <20240411072445.522731-1-alexandre.chartre@oracle.com> <7f1faa48-6252-4409-aefc-2ed2f38fb1c3@citrix.com> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 11/04/2024 10:33 am, Alexandre Chartre wrote: > > > On 4/11/24 10:43, Andrew Cooper wrote: >> On 11/04/2024 8:24 am, Alexandre Chartre wrote: >>> When a system is not affected by the BHI bug then KVM should >>> configure guests with BHI_NO to ensure they won't enable any >>> BHI mitigation. >>> >>> Signed-off-by: Alexandre Chartre >>> --- >>>   arch/x86/kvm/x86.c | 3 +++ >>>   1 file changed, 3 insertions(+) >>> >>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >>> index 984ea2089efc..f43d3c15a6b7 100644 >>> --- a/arch/x86/kvm/x86.c >>> +++ b/arch/x86/kvm/x86.c >>> @@ -1678,6 +1678,9 @@ static u64 kvm_get_arch_capabilities(void) >>>       if (!boot_cpu_has_bug(X86_BUG_GDS) || gds_ucode_mitigated()) >>>           data |= ARCH_CAP_GDS_NO; >>>   +    if (!boot_cpu_has_bug(X86_BUG_BHI)) >>> +        data |= ARCH_CAP_BHI_NO; >> >> This isn't true or safe. >> >> Linux only sets X86_BUG_BHI on a subset of affected parts. >> >> Skylake for example *is* affected by BHI.  It's just that existing >> mitigations are believed to suffice to mitigate BHI too. >> >> "you happen to be safe if you're doing something else too" doesn't >> remotely have the same meaning as "hardware doesn't have a history based >> predictor". >> > > So you mean we can't set ARCH_CAP_BHI_NO for the guest because we > don't know > if the guest will run the (other) existing mitigations which are > believed to > suffice to mitigate BHI? Correct. Also, when a VM really is migrating between different CPUs, things get far more complicated. > > The problem is that we can end up with a guest running extra BHI > mitigations > while this is not needed. Could we inform the guest that eIBRS is not > available > on the system so a Linux guest doesn't run with extra BHI mitigations? Well, that's why Intel specified some MSRs at 0x5000xxxx. Except I don't know anyone currently interested in implementing them, and I'm still not sure if they work correctly for some of the more complicated migration cases. ~Andrew