Re: [PATCH] emulated cmpxchg8b should be atomic on i386

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

On Mon, Dec 10, 2007 at 11:30:11PM +0200, Avi Kivity wrote:
> Marcelo Tosatti wrote:
> >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);
> > }
> >  
> 
> Won't the compiler complain when it sees an unreferenced label?
> 
> Also, we should handle the page boundary crossing case (probably by just 
> emulating as a write and crosssing fingers).

Pagetable writes should not cross page boundaries, so that should be fine.

------------------

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..e37c19d 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;
+
+		if (((addr + bytes - 1) & PAGE_MASK) != (addr & PAGE_MASK))
+			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);
+	}
+emul_write:
+#endif
+
 	return emulator_write_emulated(addr, new, bytes, vcpu);
 }
 

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