Re: [Xen-devel] [PATCH v2] libxc: introduce a per architecture scratch pfn for temporary grant mapping

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 21/01/15 13:25, Julien Grall wrote:
> The code to initialize the grant table in libxc uses
> xc_domain_maximum_gpfn() + 1 to get a guest pfn for mapping the grant
> frame and to initialize it.
>
> This solution has two major issues:
>     - The check of the return of xc_domain_maximum_gpfn is buggy because
>     xen_pfn_t is unsigned and in case of an error -ERRNO is returned.
>     Which is never catch with ( pfn <= 0 ).
>     - The guest memory layout maybe filled up to the end, i.e
>     xc_domain_maximum_gpfn() + 1 gives either 0 or an invalid PFN due to
>     hardware limitation.
>
> Futhermore, on ARM, xc_domain_maximum_gpfn() is not implemented and
> return -ENOSYS. This will make libxc to use always the same PFN which
> may colapse with an already mapped region (see xen/include/public/arch-arm.h
> for the layout).
>
> This patch only address the problem for ARM, the x86 version use the same
> behavior (ie xc_domain_maximum_gpfn() + 1), as I'm not familiar with Xen x86.
>
> A new function xc_core_arch_get_scratch_gpfn is introduced to be able to
> choose the gpfn per architecture.
>
> For the ARM version, we use the GUEST_GNTTAB_GUEST which is the base of
> the region by the guest to map the grant table. At the build time,
> nothing is mapped there.
>
> At the same time correctly check the return of xc_domain_maximum_gpfn
> for x86.
>
> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
> Cc: Jan Beulich <jbeulich@xxxxxxxx>
> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Cc: Roger Pau Monnà <roger.pau@xxxxxxxxxx>
> Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> Cc: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
> Cc: Wei Liu <wei.liu2@xxxxxxxxxx>

For the x86 side of things, Reviewed-by: Andrew Cooper
<andrew.cooper3@xxxxxxxxxx>

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