KVM: x86: Omit VCPU_REGS_RIP from emulator's _regs array

Omit RIP from the emulator's _regs array, which is used only for GPRs,
i.e. registers that can be referenced via ModRM and/or SIB bytes.  The
emulator uses the dedicated _eip field for RIP, and manually reads from
_eip to handle RIP-relative addressing.

To avoid an even bigger, slightly more dangerous change, hardcode the
number of GPRs to 16 for the time being even though 32-bit KVM's emulator
technically should only have 8 GPRs.  Add a TODO to address that in a
future commit.

See also the comments above the read_gpr() and write_gpr() declarations,
and obviously the handling in writeback_registers().

No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Message-Id: <20220526210817.3428868-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -247,8 +247,8 @@ enum x86_transfer_type {
 
 static ulong reg_read(struct x86_emulate_ctxt *ctxt, unsigned nr)
 {
-	if (WARN_ON_ONCE(nr >= 16))
-		nr &= 16 - 1;
+	if (WARN_ON_ONCE(nr >= NR_EMULATOR_GPRS))
+		nr &= NR_EMULATOR_GPRS - 1;
 
 	if (!(ctxt->regs_valid & (1 << nr))) {
 		ctxt->regs_valid |= 1 << nr;
@@ -259,8 +259,8 @@ static ulong reg_read(struct x86_emulate_ctxt *ctxt, unsigned nr)
 
 static ulong *reg_write(struct x86_emulate_ctxt *ctxt, unsigned nr)
 {
-	if (WARN_ON_ONCE(nr >= 16))
-		nr &= 16 - 1;
+	if (WARN_ON_ONCE(nr >= NR_EMULATOR_GPRS))
+		nr &= NR_EMULATOR_GPRS - 1;
 
 	ctxt->regs_valid |= 1 << nr;
 	ctxt->regs_dirty |= 1 << nr;
@@ -278,7 +278,7 @@ static void writeback_registers(struct x86_emulate_ctxt *ctxt)
 	unsigned long dirty = ctxt->regs_dirty;
 	unsigned reg;
 
-	for_each_set_bit(reg, &dirty, 16)
+	for_each_set_bit(reg, &dirty, NR_EMULATOR_GPRS)
 		ctxt->ops->write_gpr(ctxt, reg, ctxt->_regs[reg]);
 }
 

arch/x86/kvm/kvm_emulate.h

@@ -301,6 +301,17 @@ struct fastop;
 
 typedef void (*fastop_t)(struct fastop *);
 
+/*
+ * The emulator's _regs array tracks only the GPRs, i.e. excludes RIP.  RIP is
+ * 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.
+ */
+#define NR_EMULATOR_GPRS	16
+
 struct x86_emulate_ctxt {
 	void *vcpu;
 	const struct x86_emulate_ops *ops;
@@ -363,7 +374,7 @@ struct x86_emulate_ctxt {
 	struct operand src2;
 	struct operand dst;
 	struct operand memop;
-	unsigned long _regs[NR_VCPU_REGS];
+	unsigned long _regs[NR_EMULATOR_GPRS];
 	struct operand *memopp;
 	struct fetch_cache fetch;
 	struct read_cache io_read;