linux-kernel - Re: [PATCH 4/4] KVM: x86: Use 16-bit fields to track dirty/valid emulator GPRs

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Message-ID: <202205260840.3B83593@keescook>
Date:   Thu, 26 May 2022 08:41:04 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Robert Dinse <nanook@...imo.com>
Subject: Re: [PATCH 4/4] KVM: x86: Use 16-bit fields to track dirty/valid
 emulator GPRs

On Wed, May 25, 2022 at 10:26:04PM +0000, Sean Christopherson wrote:
> Use a u16 instead of a u32 to track the dirty/valid status of GPRs in the
> emulator.  Unlike struct kvm_vcpu_arch, x86_emulate_ctxt tracks only the
> "true" GPRs, i.e. doesn't include RIP in its array, and so only needs to
> track 16 registers.
> 
> Note, having 16 GPRs is a fundamental property of x86-64 and will not
> change barring a massive architecture update.  Legacy x86 ModRM and SIB
> encodings use 3 bits for GPRs, i.e. support 8 registers.  x86-64 uses a
> single bit in the REX prefix for each possible reference type to double
> the number of supported GPRs to 16 registers (4 bits).
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook