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Re: [Xen-devel] [PATCH] x86: Don't crash when mapping a page using EFI runtime page tables

On 05/18/2015 03:58 PM, Jan Beulich wrote:
On 15.05.15 at 18:08, <ross.lagerwall@xxxxxxxxxx> wrote:
--- a/xen/arch/x86/domain_page.c
+++ b/xen/arch/x86/domain_page.c
@@ -32,20 +32,25 @@ static inline struct vcpu *mapcache_current_vcpu(void)
          return NULL;

+     * When using efi runtime page tables, we have the equivalent of the idle
+     * domain's page tables but current may point at another domain's VCPU.
+     * Return NULL as though current is not properly set up yet.
+     */
+    if ( efi_enabled && read_cr3() == efi_rs_page_table() )
+        return NULL;

I'm okay with the patch in principle; what worries me is the CR3 read
that is now going to be necessary even in non-debug builds. With
this code being the only user of efi_rs_page_table(), I wonder if it
wouldn't make sense to alter that function to return non-zero only
when spin_is_locked(&efi_rs_lock), and then alter the code above
such that the CR3 read would happen only when we got a non-zero
value back.

mapcache_current_vcpu() appears to be called from IRQ-enabled and IRQ-disabled callers which prevents us from using the spinlock. Is reading cr3 that bad, given that this is a slow path anyway?

And with that I would then also like to fold the potential double CR3
reads in debug builds to at most one.

CR3 needs to be re-read after the lazy context switch to the idle domain is completed.

Ross Lagerwall

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