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

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Message-Id: <20220525222604.2810054-5-seanjc@google.com>
Date:   Wed, 25 May 2022 22:26:04 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.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>,
        Kees Cook <keescook@...omium.org>
Subject: [PATCH 4/4] KVM: x86: Use 16-bit fields to track dirty/valid emulator GPRs

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>
---
 arch/x86/kvm/emulate.c     | 3 +++
 arch/x86/kvm/kvm_emulate.h | 4 ++--
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index dd1bf116a9ed..afb115b6a5a4 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -262,6 +262,9 @@ static ulong *reg_write(struct x86_emulate_ctxt *ctxt, unsigned nr)
 	if (WARN_ON_ONCE(nr >= NR_EMULATOR_GPRS))
 		nr &= NR_EMULATOR_GPRS - 1;
 
+	BUILD_BUG_ON(sizeof(ctxt->regs_dirty) * BITS_PER_BYTE < NR_EMULATOR_GPRS);
+	BUILD_BUG_ON(sizeof(ctxt->regs_valid) * BITS_PER_BYTE < NR_EMULATOR_GPRS);
+
 	ctxt->regs_valid |= 1 << nr;
 	ctxt->regs_dirty |= 1 << nr;
 	return &ctxt->_regs[nr];
diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
index bdd4e9865ca9..fbe87ba78163 100644
--- a/arch/x86/kvm/kvm_emulate.h
+++ b/arch/x86/kvm/kvm_emulate.h
@@ -353,9 +353,9 @@ struct x86_emulate_ctxt {
 	u8 lock_prefix;
 	u8 rep_prefix;
 	/* bitmaps of registers in _regs[] that can be read */
-	u32 regs_valid;
+	u16 regs_valid;
 	/* bitmaps of registers in _regs[] that have been written */
-	u32 regs_dirty;
+	u16 regs_dirty;
 	/* modrm */
 	u8 modrm;
 	u8 modrm_mod;
-- 
2.36.1.124.g0e6072fb45-goog