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, maxing out at 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).

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220526210817.3428868-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
index bc3f8295c8c8..3a65d6ea7fe6 100644
--- a/arch/x86/kvm/kvm_emulate.h
+++ b/arch/x86/kvm/kvm_emulate.h
@@ -356,9 +356,9 @@ 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;