[PATCH v3 0/2] epte_get_entry_emt() modifications

[PATCH v3 0/2] epte_get_entry_emt() modifications

[Paul Durrant]

From: Paul Durrant <pdurrant@amazon.com>

Paul Durrant (2):
  x86/hvm: set 'ipat' in EPT for special pages
  x86/hvm: simplify 'mmio_direct' check in epte_get_entry_emt()

 xen/arch/x86/hvm/mtrr.c | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)
---
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: Wei Liu <wl@xen.org>
-- 
2.20.1

[PATCH v3 1/2] x86/hvm: set 'ipat' in EPT for special pages

[Paul Durrant]

From: Paul Durrant <pdurrant@amazon.com>

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@amazon.com>
---
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Wei Liu <wl@xen.org>
Cc: "Roger Pau Monné" <roger.pau@citrix.com>

v3:
 - dropping Jan's R-b
 - cope with order > 0
---
 xen/arch/x86/hvm/mtrr.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/xen/arch/x86/hvm/mtrr.c b/xen/arch/x86/hvm/mtrr.c
index 511c3be1c8..26721f6ee7 100644
--- a/xen/arch/x86/hvm/mtrr.c
+++ b/xen/arch/x86/hvm/mtrr.c
@@ -836,6 +836,17 @@ 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 )
     {
-- 
2.20.1

RE: [PATCH v3 1/2] x86/hvm: set 'ipat' in EPT for special pages

[Durrant, Paul]

> -----Original Message-----
> From: Paul Durrant <paul@xen.org>
> Sent: 31 July 2020 15:26
> To: xen-devel@lists.xenproject.org
> Cc: Durrant, Paul <pdurrant@amazon.co.uk>; Jan Beulich <jbeulich@suse.com>; Andrew Cooper
> <andrew.cooper3@citrix.com>; Wei Liu <wl@xen.org>; Roger Pau Monné <roger.pau@citrix.com>
> Subject: [EXTERNAL] [PATCH v3 1/2] x86/hvm: set 'ipat' in EPT for special pages
> 
> CAUTION: This email originated from outside of the organization. Do not click links or open
> attachments unless you can confirm the sender and know the content is safe.
> 
> 
> 
> From: Paul Durrant <pdurrant@amazon.com>
> 
> 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@amazon.com>

This is missing a hunk. I'll send v4.

  Paul

> ---
> Cc: Jan Beulich <jbeulich@suse.com>
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> Cc: Wei Liu <wl@xen.org>
> Cc: "Roger Pau Monné" <roger.pau@citrix.com>
> 
> v3:
>  - dropping Jan's R-b
>  - cope with order > 0
> ---
>  xen/arch/x86/hvm/mtrr.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/xen/arch/x86/hvm/mtrr.c b/xen/arch/x86/hvm/mtrr.c
> index 511c3be1c8..26721f6ee7 100644
> --- a/xen/arch/x86/hvm/mtrr.c
> +++ b/xen/arch/x86/hvm/mtrr.c
> @@ -836,6 +836,17 @@ 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 )
>      {
> --
> 2.20.1

[PATCH v3 2/2] x86/hvm: simplify 'mmio_direct' check in epte_get_entry_emt()

[Paul Durrant]

From: Paul Durrant <pdurrant@amazon.com>

Re-factor the code to take advantage of the fact that the APIC access page is
a 'special' page. The VMX code is left alone and hence the APIC access page is
still inserted into the P2M with type p2m_mmio_direct. This is left alone as it
is not obvious there is another suitable type to use, and the necessary
re-ordering in epte_get_entry_emt() is straightforward.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Paul Durrant <pdurrant@amazon.com>
---
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Wei Liu <wl@xen.org>
Cc: "Roger Pau Monné" <roger.pau@citrix.com>

v2:
 - New in v2

v3:
 - Re-base
 - Expand commit comment
---
 xen/arch/x86/hvm/mtrr.c | 15 ++++-----------
 1 file changed, 4 insertions(+), 11 deletions(-)

diff --git a/xen/arch/x86/hvm/mtrr.c b/xen/arch/x86/hvm/mtrr.c
index 26721f6ee7..cfdbcbfef1 100644
--- a/xen/arch/x86/hvm/mtrr.c
+++ b/xen/arch/x86/hvm/mtrr.c
@@ -814,23 +814,13 @@ int epte_get_entry_emt(struct domain *d, unsigned long gfn, mfn_t mfn,
         return -1;
     }
 
-    if ( direct_mmio )
-    {
-        if ( (mfn_x(mfn) ^ mfn_x(d->arch.hvm.vmx.apic_access_mfn)) >> order )
-            return MTRR_TYPE_UNCACHABLE;
-        if ( order )
-            return -1;
-        *ipat = 1;
-        return MTRR_TYPE_WRBACK;
-    }
-
     if ( !mfn_valid(mfn) )
     {
         *ipat = 1;
         return MTRR_TYPE_UNCACHABLE;
     }
 
-    if ( !is_iommu_enabled(d) && !cache_flush_permitted(d) )
+    if ( !direct_mmio && !is_iommu_enabled(d) && !cache_flush_permitted(d) )
     {
         *ipat = 1;
         return MTRR_TYPE_WRBACK;
@@ -847,6 +837,9 @@ int epte_get_entry_emt(struct domain *d, unsigned long gfn, mfn_t mfn,
         }
     }
 
+    if ( direct_mmio )
+        return MTRR_TYPE_UNCACHABLE;
+
     gmtrr_mtype = hvm_get_mem_pinned_cacheattr(d, _gfn(gfn), order);
     if ( gmtrr_mtype >= 0 )
     {
-- 
2.20.1