[RFC,v6,92/92] kvm: x86: fallback to the single-step on multipage CMPXCHG emulation

Commit Message

Adalbert Lazăr Aug. 9, 2019, 4 p.m. UTC

From: Mihai Donțu <[e-mail]>

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 <[e-mail]>
Signed-off-by: Adalbert Lazăr <[e-mail]>
---
 arch/x86/kvm/x86.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

Patch

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);