[PATCH] emulated cmpxchg8b should be atomic on i386

[Marcelo Tosatti <marcelo-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org>]

Emulate cmpxchg8b atomically on i386. This is required to avoid a guest
pte walker from seeing a splitted write.

Signed-off-by: Marcelo Tosatti <mtosatti-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>

diff --git a/drivers/kvm/x86.c b/drivers/kvm/x86.c
index 9db4e32..d58d795 100644
--- a/drivers/kvm/x86.c
+++ b/drivers/kvm/x86.c
@@ -1674,6 +1674,33 @@ static int emulator_cmpxchg_emulated(uns
 		reported = 1;
 		printk(KERN_WARNING "kvm: emulating exchange as write\n");
 	}
+#ifndef CONFIG_X86_64
+	/* guests cmpxchg8b have to be emulated atomically */
+	if (bytes == 8) {
+		gpa_t gpa = vcpu->mmu.gva_to_gpa(vcpu, addr);
+		struct page *page;
+		char *addr;
+		u64 *val;
+
+		if (gpa == UNMAPPED_GVA ||
+	    	   (gpa & PAGE_MASK) == APIC_DEFAULT_PHYS_BASE)
+			goto emul_write;
+
+		val = (u64 *)new;
+		page = gfn_to_page(page, gpa >> PAGE_SHIFT);
+		addr = kmap_atomic(page, KM_USER0);
+		addr += offset_in_page(gpa);
+
+		set_64bit((unsigned long long *)addr, val);
+
+		kunmap_atomic(page, KM_USER0);
+		kvm_release_page_dirty(page);
+
+		return X86EMUL_CONTINUE;
+	}
+#endif
+
+emul_write:
 	return emulator_write_emulated(addr, new, bytes, vcpu);
 }
 

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