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

["Durrant, Paul" <pdurrant@amazon.co.uk>]

> -----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
> 
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> 
> 
> 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