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>

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index c74c0fd..5aaf1d1 100644 (file)
--- 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 bc3f829..3a65d6e 100644 (file)
--- 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;