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);
}
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