[Xen-changelog] [xen stable-4.12] x86/tlbflush: do not toggle the PGE CR4 bit unless necessary

[patchbot@xxxxxxx]

commit 212b8500cb394b3a664655f79ca0bdcb31246ff7
Author:     Roger Pau Monné <roger.pau@xxxxxxxxxx>
AuthorDate: Fri Dec 6 12:49:45 2019 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Dec 6 12:49:45 2019 +0100

    x86/tlbflush: do not toggle the PGE CR4 bit unless necessary
    
    When PCID is not available Xen does a full tlbflush by toggling the
    PGE bit in CR4. This is not necessary if PGE is not enabled, since a
    flush can be performed by writing to CR3 in that case.
    
    Change the code in do_tlb_flush to only toggle the PGE bit in CR4 if
    it's already enabled, otherwise do the tlb flush by writing to CR3.
    This is relevant when running virtualized, since hypervisors don't
    usually trap accesses to CR3 when using hardware assisted paging, but
    do trap accesses to CR4 specially on AMD hardware, which makes such
    accesses much more expensive.
    
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: b5087a31efee7a4e34c958b88671ac6669501b09
    master date: 2019-12-03 14:15:35 +0100
---
 xen/arch/x86/flushtlb.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/flushtlb.c b/xen/arch/x86/flushtlb.c
index 4004129c49..6829890fe3 100644
--- a/xen/arch/x86/flushtlb.c
+++ b/xen/arch/x86/flushtlb.c
@@ -78,7 +78,7 @@ static void post_flush(u32 t)
 
 static void do_tlb_flush(void)
 {
-    unsigned long flags;
+    unsigned long flags, cr4;
     u32 t;
 
     /* This non-reentrant function is sometimes called in interrupt context. */
@@ -88,13 +88,13 @@ static void do_tlb_flush(void)
 
     if ( use_invpcid )
         invpcid_flush_all();
-    else
+    else if ( (cr4 = read_cr4()) & X86_CR4_PGE )
     {
-        unsigned long cr4 = read_cr4();
-
-        write_cr4(cr4 ^ X86_CR4_PGE);
+        write_cr4(cr4 & ~X86_CR4_PGE);
         write_cr4(cr4);
     }
+    else
+        write_cr3(read_cr3());
 
     post_flush(t);
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.12

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