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Date: Mon, 15 Apr 2024 10:17:06 -0700
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Subject: Re: [PATCH] KVM: x86: Set BHI_NO in guest when host is not affected
 by BHI
To: Alexandre Chartre <alexandre.chartre@oracle.com>,
 Chao Gao <chao.gao@intel.com>, Andrew Cooper <andrew.cooper3@citrix.com>
Cc: x86@kernel.org, kvm@vger.kernel.org, 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>
 <caa51938-c587-4403-a9cd-16e8b585bc13@oracle.com>
 <d47dcc77-3c8b-4f78-954a-a64d3a905224@citrix.com>
 <ZhfGHpAz7W7d/pSa@chao-email>
 <95902795-0c2c-43cc-8d87-89302a2eed2b@oracle.com>
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From: Dave Hansen <dave.hansen@intel.com>
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In-Reply-To: <2af16cb4-32ed-4b91-872b-f0cc9ed92e59@oracle.com>
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> +       /*
> +        * The following Intel CPUs are affected by BHI, but they don't have
> +        * the eIBRS feature. In that case, the default Spectre v2 mitigations
> +        * are enough to also mitigate BHI. We mark these CPUs with NO_BHI so
> +        * that X86_BUG_BHI doesn't get set and no extra BHI mitigation is
> +        * enabled.
> +        *
> +        * This avoids guest VMs from enabling extra BHI mitigation when this
> +        * is not needed. For guest, X86_BUG_BHI is never set for CPUs which
> +        * don't have the eIBRS feature. But this doesn't happen in guest VMs
> +        * as the virtualization can hide the eIBRS feature.
> +        */
> +       VULNWL_INTEL(IVYBRIDGE_X,               NO_BHI),
> +       VULNWL_INTEL(HASWELL_X,                 NO_BHI),
> +       VULNWL_INTEL(BROADWELL_X,               NO_BHI),
> +       VULNWL_INTEL(SKYLAKE_X,                 NO_BHI),
> +       VULNWL_INTEL(SKYLAKE_X,                 NO_BHI),

Isn't this at odds with the existing comment?

        /* When virtualized, eIBRS could be hidden, assume vulnerable */

Because it seems now that we've got two relatively conflicting pieces of
vulnerability information when running under a hypervisor.