[xen staging] x86/mm: avoid playing with directmap when self-snoop can be relied upon

[patchbot@xxxxxxx]

commit f9c53bd9eeff5d93e63665d67265dd27155e905e
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Fri Oct 23 10:06:20 2020 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Oct 23 10:06:20 2020 +0200

    x86/mm: avoid playing with directmap when self-snoop can be relied upon
    
    The set of systems affected by XSA-345 would have been smaller is we had
    this in place already: When the processor is capable of dealing with
    mismatched cacheability, there's no extra work we need to carry out.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/mm.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 6d2262a3f0..b2f35b3e7d 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -795,6 +795,9 @@ static int update_xen_mappings(unsigned long mfn, unsigned 
int cacheattr)
     unsigned long xen_va =
         XEN_VIRT_START + ((mfn - PFN_DOWN(xen_phys_start)) << PAGE_SHIFT);
 
+    if ( boot_cpu_has(X86_FEATURE_XEN_SELFSNOOP) )
+        return 0;
+
     if ( unlikely(alias) && cacheattr )
         err = map_pages_to_xen(xen_va, _mfn(mfn), 1, 0);
     if ( !err )
@@ -802,6 +805,7 @@ static int update_xen_mappings(unsigned long mfn, unsigned 
int cacheattr)
                      PAGE_HYPERVISOR | cacheattr_to_pte_flags(cacheattr));
     if ( unlikely(alias) && !cacheattr && !err )
         err = map_pages_to_xen(xen_va, _mfn(mfn), 1, PAGE_HYPERVISOR);
+
     return err;
 }
 
--
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