[PATCH v2 3/8] KVM: x86: Omit VCPU_REGS_RIP from emulator's _regs array

Sean Christopherson
2022-05-26T21:24:31Z

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>
---
arch/x86/kvm/emulate.c | 10 +++++-----
arch/x86/kvm/kvm_emulate.h | 13 ++++++++++++-
2 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index c58366ae4da2..c74c0fd3b860 100644
--- a/arch/x86/kvm/emulate.c
+++ b/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]);
}

diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
index 8dff25d267b7..bc3f8295c8c8 100644
--- a/arch/x86/kvm/kvm_emulate.h
+++ b/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;
--
2.36.1.255.ge46751e96f-goog