linux-kernel - Re: [PATCH 0/2] KVM: x86: Propagate AMD-specific IBRS bits to guests

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Date:   Tue, 7 Mar 2023 10:49:22 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Takahiro Itazuri <itazur@...zon.com>,
        dave.hansen@...ux.intel.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, mingo@...hat.com, tglx@...utronix.de,
        x86@...nel.org, zulinx86@...il.com
Subject: Re: [PATCH 0/2] KVM: x86: Propagate AMD-specific IBRS bits to guests

On Mon, Mar 06, 2023, Borislav Petkov wrote:
> On Mon, Mar 06, 2023 at 10:47:18PM +0100, Paolo Bonzini wrote:
> > It's very rare that KVM can provide a CPUID feature if the kernel has
> > masked it,
> 
> I'm talking about pure hw feature bits which don't need any enablement.
> Like AVX512 insns subset support or something else which the hw does
> without the need for the kernel.
> 
> Those should be KVM-only if baremetal doesn't use them.

I don't see what such a rule buys us beyond complexity and, IMO, unnecessary
maintenance burden.  As Paolo pointed out, when there's an existing word, the
only "cost" is the existence of the #define.  The bit is still present in the
capabilities, and KVM relies on this!  And as mentioned in the tangent about
reworking cpufeatures[*], I get a _lot_ of value out of cpufeatures.h being fully
populated for known bits (in defined words).

Forcing KVM to #define bits in reverse_cpuid.h just because the kernel doesn't
need the macro will inevitably lead to confusion for KVM developers, both when
writing new code and when reading existing code.

[*] https://lore.kernel.org/all/Y8nhtjFcsB63UsmQ@google.com