[xen master] x86/pv: Drop FLUSH_TLB_GLOBAL in do_mmu_update() for XPTI

[patchbot@xxxxxxx]

commit 055e1c3a3d95b1e753148369fbc4ba48782dd602
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Thu Oct 22 11:28:58 2020 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Oct 27 17:39:11 2020 +0000

    x86/pv: Drop FLUSH_TLB_GLOBAL in do_mmu_update() for XPTI
    
    c/s 9d1d31ad9498 "x86: slightly reduce Meltdown band-aid overhead" removed 
the
    use of Global TLB flushes on the Xen entry path, but added a 
FLUSH_TLB_GLOBAL
    to the L4 path in do_mmu_update().
    
    However, this was unnecessary.
    
    It is the guests responsibility to perform appropriate TLB flushing if the 
L4
    modification altered an established mapping in a flush-relevant way.  In 
this
    case, an MMUEXT_OP hypercall will follow.  The case which Xen needs to cover
    is when new mappings are created, and the resync on the exit-to-guest path
    covers this correctly.
    
    There is a corner case with multiple vCPUs in hypercalls at the same time,
    which 9d1d31ad9498 changed, and this patch changes back to its original XPTI
    behaviour.
    
    Architecturally, established TLB entries can continue to be used until the
    broadcast flush has completed.  Therefore, even with concurrent hypercalls,
    the guest cannot depend on older mappings not being used until an MMUEXT_OP
    hypercall completes.  Xen's implementation of guest-initiated flushes will
    take correct effect on top of an in-progress hypercall, picking up new 
mapping
    setting before the other vCPU's MMUEXT_OP completes.
    
    Note: The correctness of this change is not impacted by whether XPTI uses
    global mappings or not.  Correctness there depends on the behaviour of Xen 
on
    the entry/exit paths when switching two/from the XPTI "shadow" pagetables.
    
    This is (not really) XSA-286 (but necessary to simplify the logic).
    
    Fixes: 9d1d31ad9498 ("x86: slightly reduce Meltdown band-aid overhead")
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/mm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index b2f35b3e7d..38168189aa 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -4188,7 +4188,7 @@ long do_mmu_update(
 
         cpumask_andnot(mask, pt_owner->dirty_cpumask, cpumask_of(cpu));
         if ( !cpumask_empty(mask) )
-            flush_mask(mask, FLUSH_TLB_GLOBAL | FLUSH_ROOT_PGTBL);
+            flush_mask(mask, FLUSH_ROOT_PGTBL);
     }
 
     perfc_add(num_page_updates, i);
--
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