[PATCH v2 4/8] KVM: x86: Use 16-bit fields to track dirty/valid emulator GPRs

Sean Christopherson
2022-05-26T21:24:29Z

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>
---
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 c74c0fd3b860..5aaf1d1200af 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 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;
--
2.36.1.255.ge46751e96f-goog