Re: [PATCH] Use cmpxchg for pte updates on walk_addr()

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

On Fri, Dec 07, 2007 at 07:06:26AM +0200, Avi Kivity wrote:
> Marcelo Tosatti wrote:
> >Right, patch at end of the message restarts the process if the pte
> >changes under the walker. The goto is pretty ugly, but I fail to see any
> >elegant way of doing that. Ideas?
> >
> >  
> 
> goto is fine for that.  But there's a subtle livelock here: suppose vcpu 
> 0 is in guest mode with continuously updating a memory location.  vcpu 1 
> is faulting with that memory location acting as a pte.  While we're in 
> kernel mode, we aren't responding to signals like we should; so we need 
> to abort the walk and let the guest retry; that way we go through the 
> signal_pending() check.
> 
> However, this is an intrusive change, so let's start with the goto and 
> drop it later in favor or an abort.
> 
> >>>@@ -1510,6 +1510,9 @@ static int emulator_write_phys(struct kvm_vcpu 
> >>>*vcpu, gpa_t gpa,
> >>> {
> >>> 	int ret;
> >>> 
> >>>+	/* No need for kvm_cmpxchg_guest_pte here, its the guest 
> >>>+ 	 * responsability to synchronize pte updates and page faults.
> >>>+	 */
> >>> 	ret = kvm_write_guest(vcpu->kvm, gpa, val, bytes);
> >>> 	if (ret < 0)
> >>> 		return 0;
> >>>      
> >>Hmm.  What if an i386 pae guest carefully uses cmpxchg8b to atomically 
> >>set a pte?  kvm_write_guest() doesn't guarantee atomicity, so an 
> >>intended atomic write can be seen splitted by the guest walker doing a 
> >>concurrent walk.
> >>    
> >
> >True, an atomic write is needed... a separate patch for that seems more
> >appropriate.
> >
> >
> >  
> 
> Yes.

Hi Avi,

How about the following

diff --git a/drivers/kvm/x86.c b/drivers/kvm/x86.c
index c70ac33..8678651 100644
--- a/drivers/kvm/x86.c
+++ b/drivers/kvm/x86.c
@@ -25,6 +25,7 @@
 #include <linux/vmalloc.h>
 #include <linux/module.h>
 #include <linux/mman.h>
+#include <linux/highmem.h>
 
 #include <asm/uaccess.h>
 #include <asm/msr.h>
@@ -1589,6 +1590,34 @@ static int emulator_cmpxchg_emulated(unsigned long addr,
 		reported = 1;
 		printk(KERN_WARNING "kvm: emulating exchange as write\n");
 	}
+
+	/* 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(vcpu->kvm, gpa >> PAGE_SHIFT);
+		addr = kmap_atomic(page, KM_USER0);
+		addr += offset_in_page(gpa);
+
+#ifdef CONFIG_X86_64
+		set_64bit((unsigned long *)addr, *val);
+#else
+		set_64bit((unsigned long long *)addr, *val);
+#endif
+		kunmap_atomic(page, KM_USER0);
+		kvm_release_page_dirty(page);
+			
+	}
+
+emul_write:
 	return emulator_write_emulated(addr, new, bytes, vcpu);
 }
 

-------------------------------------------------------------------------
SF.Net email is sponsored by:
Check out the new SourceForge.net Marketplace.
It's the best place to buy or sell services for
just about anything Open Source.
http://sourceforge.net/services/buy/index.php