[xen master] x86/hvm: set 'ipat' in EPT for special pages

[patchbot@xxxxxxx]

commit ca24b2ffdbd9a25b2d313a547ccbe97baf3e5a8d
Author:     Paul Durrant <pdurrant@xxxxxxxxxx>
AuthorDate: Fri Jul 31 17:42:47 2020 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Jul 31 17:42:47 2020 +0200

    x86/hvm: set 'ipat' in EPT for special pages
    
    All non-MMIO ranges (i.e those not mapping real device MMIO regions) that
    map valid MFNs are normally marked MTRR_TYPE_WRBACK and 'ipat' is set. Hence
    when PV drivers running in a guest populate the BAR space of the Xen 
Platform
    PCI Device with pages such as the Shared Info page or Grant Table pages,
    accesses to these pages will be cachable.
    
    However, should IOMMU mappings be enabled be enabled for the guest then 
these
    accesses become uncachable. This has a substantial negative effect on I/O
    throughput of PV devices. Arguably PV drivers should bot be using BAR space 
to
    host the Shared Info and Grant Table pages but it is currently commonplace 
for
    them to do this and so this problem needs mitigation. Hence this patch makes
    sure the 'ipat' bit is set for any special page regardless of where in GFN
    space it is mapped.
    
    NOTE: Clearly this mitigation only applies to Intel EPT. It is not obvious
          that there is any similar mitigation possible for AMD NPT. Downstreams
          such as Citrix XenServer have been carrying a patch similar to this 
for
          several releases though.
    
    Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/hvm/mtrr.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/xen/arch/x86/hvm/mtrr.c b/xen/arch/x86/hvm/mtrr.c
index 511c3be1c8..2bd64e8025 100644
--- a/xen/arch/x86/hvm/mtrr.c
+++ b/xen/arch/x86/hvm/mtrr.c
@@ -794,6 +794,7 @@ int epte_get_entry_emt(struct domain *d, unsigned long gfn, 
mfn_t mfn,
 {
     int gmtrr_mtype, hmtrr_mtype;
     struct vcpu *v = current;
+    unsigned long i;
 
     *ipat = 0;
 
@@ -836,6 +837,17 @@ int epte_get_entry_emt(struct domain *d, unsigned long 
gfn, mfn_t mfn,
         return MTRR_TYPE_WRBACK;
     }
 
+    for ( i = 0; i < (1ul << order); i++ )
+    {
+        if ( is_special_page(mfn_to_page(mfn_add(mfn, i))) )
+        {
+            if ( order )
+                return -1;
+            *ipat = 1;
+            return MTRR_TYPE_WRBACK;
+        }
+    }
+
     gmtrr_mtype = hvm_get_mem_pinned_cacheattr(d, _gfn(gfn), order);
     if ( gmtrr_mtype >= 0 )
     {
--
generated by git-patchbot for /home/xen/git/xen.git#master