Re: [Xen-devel] [PATCH v3 4/5] x86/HVM: fix pinned cache attribute handling

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Monday, April 07, 2014 6:10 PM
> 
> - make sure UC- is only used for PAT purposes (MTRRs and hence EPT
>   don't have this type)
> - add order input to "get", and properly handle conflict case (forcing
>   an EPT page split)

could you elaborate a bit how an EPT page split is forced with this change?

> - properly detect (and refuse) overlaps during "set"
> - properly use RCU constructs
> - support deleting ranges through a special type input to "set"
> - set ignore-PAT flag in epte_get_entry_emt() when "get" succeeds
> - set "get" output to ~0 (invalid) rather than 0 (UC) on error (the
>   caller shouldn't be looking at it anyway)
> - move struct hvm_mem_pinned_cacheattr_range from header to C file
>   (used only there)
> 
> Note that the code (before and after this change) implies the GFN
> ranges passed to the hypercall to be inclusive, which is in contrast
> to the sole current user in qemu (all variants). It is not clear to me
> at which layer (qemu, libxc, hypervisor) this would best be fixed.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Reviewed-by: Tim Deegan <tim@xxxxxxx>
> 
> @@ -732,8 +801,14 @@ int epte_get_entry_emt(struct domain *d,
>      if ( !mfn_valid(mfn_x(mfn)) )
>          return MTRR_TYPE_UNCACHABLE;
> 
> -    if ( hvm_get_mem_pinned_cacheattr(d, gfn, &type) )
> -        return type;
> +    switch ( hvm_get_mem_pinned_cacheattr(d, gfn, order, &type) )
> +    {
> +    case 1:
> +        *ipat = 1;
> +        return type != PAT_TYPE_UC_MINUS ? type :
> PAT_TYPE_UNCACHABLE;
> +    case -1:
> +        return -1;
> +    }

what does '-1' mean for a cache type? I didn't see callers check this
value, e.g:

    /* Construct the new entry, and then write it once */
    new_entry.emt = epte_get_entry_emt(p2m->domain, gfn, mfn, &ipat,
                                    direct_mmio);

Thanks
Kevin

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