[PATCH v2 5/8] KVM: x86: Reduce the number of emulator GPRs to '8' for 32-bit KVM

Sean Christopherson
2022-05-26T21:24:38Z

Reduce the number of GPRs emulated by 32-bit KVM from 16 to 8. KVM does
not support emulating 64-bit mode on 32-bit host kernels, and so should
never generate accesses to R8-15.

Opportunistically use NR_EMULATOR_GPRS in rsm_load_state_{32,64}() now
that it is precise and accurate for both flavors.

Wrap the definition with full #ifdef ugliness; sadly, IS_ENABLED()
doesn't guarantee a compile-time constant as far as BUILD_BUG_ON() is
concerned.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/emulate.c | 4 ++--
arch/x86/kvm/kvm_emulate.h | 7 ++++---
2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 5aaf1d1200af..77161f57c8d3 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -2441,7 +2441,7 @@ static int rsm_load_state_32(struct x86_emulate_ctxt *ctxt,
ctxt->eflags = GET_SMSTATE(u32, smstate, 0x7ff4) | X86_EFLAGS_FIXED;
ctxt->_eip = GET_SMSTATE(u32, smstate, 0x7ff0);

- for (i = 0; i < 8; i++)
+ for (i = 0; i < NR_EMULATOR_GPRS; i++)
*reg_write(ctxt, i) = GET_SMSTATE(u32, smstate, 0x7fd0 + i * 4);

val = GET_SMSTATE(u32, smstate, 0x7fcc);
@@ -2498,7 +2498,7 @@ static int rsm_load_state_64(struct x86_emulate_ctxt *ctxt,
u16 selector;
int i, r;

- for (i = 0; i < 16; i++)
+ for (i = 0; i < NR_EMULATOR_GPRS; i++)
*reg_write(ctxt, i) = GET_SMSTATE(u64, smstate, 0x7ff8 - i * 8);

ctxt->_eip = GET_SMSTATE(u64, smstate, 0x7f78);
diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
index 3a65d6ea7fe6..034c845b3c63 100644
--- a/arch/x86/kvm/kvm_emulate.h
+++ b/arch/x86/kvm/kvm_emulate.h
@@ -306,11 +306,12 @@ typedef void (*fastop_t)(struct fastop *);
* tracked/accessed via _eip, and except for RIP relative addressing, which
* also uses _eip, RIP cannot be a register operand nor can it be an operand in
* a ModRM or SIB byte.
- *
- * TODO: this is technically wrong for 32-bit KVM, which only supports 8 GPRs;
- * R8-R15 don't exist.
*/
+#ifdef CONFIG_X86_64
#define NR_EMULATOR_GPRS 16
+#else
+#define NR_EMULATOR_GPRS 8
+#endif

struct x86_emulate_ctxt {
void *vcpu;
--
2.36.1.255.ge46751e96f-goog