From: Mihai Donțu <mdontu@xxxxxxxxxxxxxxx>
There are cases where we need to emulate a CMPXCHG that touches two
pages (4 in one and another 4 in the next, for example). Because it
is not easy to map two pages in the kernel so that we can directly
execute the exchange instruction, we fallback to single-stepping.
Luckly, this is an uncommon occurrence making the overhead of the
single-step mechanism acceptable.
Signed-off-by: Mihai Donțu <mdontu@xxxxxxxxxxxxxxx>
Signed-off-by: Adalbert Lazăr <alazar@xxxxxxxxxxxxxxx>
---
arch/x86/kvm/x86.c | 15 +++++++++------
1 file changed, 9 insertions(+), 6 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0e904782d303..e283b074db26 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5671,6 +5671,12 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
#define CMPXCHG_MAX_BYTES 8
#endif
+ gpa = kvm_mmu_gva_to_gpa_write(vcpu, addr, NULL);
+
+ if (gpa == UNMAPPED_GVA ||
+ (gpa & PAGE_MASK) == APIC_DEFAULT_PHYS_BASE)
+ goto emul_write;
+
/* guests cmpxchg{8,16}b have to be emulated atomically */
if (bytes > CMPXCHG_MAX_BYTES || (bytes & (bytes - 1)))
goto emul_write;
@@ -5678,12 +5684,6 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
if (bytes == 16 && !system_has_cmpxchg_double())
goto emul_write;
- gpa = kvm_mmu_gva_to_gpa_write(vcpu, addr, NULL);
-
- if (gpa == UNMAPPED_GVA ||
- (gpa & PAGE_MASK) == APIC_DEFAULT_PHYS_BASE)
- goto emul_write;
-
if (((gpa + bytes - 1) & PAGE_MASK) != (gpa & PAGE_MASK))
goto emul_write;
@@ -5772,6 +5772,9 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
return X86EMUL_CONTINUE;
emul_write:
+ if (kvmi_tracked_gfn(vcpu, gpa >> PAGE_SHIFT))
+ return X86EMUL_UNHANDLEABLE;
+
printk_once(KERN_WARNING "kvm: emulating exchange as write\n");
return emulator_write_emulated(ctxt, addr, new, bytes, exception);
