Re: [Xen-devel] [PATCH v2 REPOST 03/12] x86/mm: add HYPERVISOR_memory_op to acquire guest resources

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 29.08.17 at 10:32, <Paul.Durrant@xxxxxxxxxx> wrote:
>> From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
>> Sent: 28 August 2017 16:01
>> On Tue, Aug 22, 2017 at 03:50:57PM +0100, Paul Durrant wrote:
>> > +
>> > +/*
>> > + * Get the pages for a particular guest resource, so that they can be
>> > + * mapped directly by a tools domain.
>> > + */
>> > +#define XENMEM_acquire_resource 28
>> > +struct xen_mem_acquire_resource {
>> > +    /* IN - the domain whose resource is to be mapped */
>> > +    domid_t domid;
>> > +    /* IN - the type of resource (defined below) */
>> > +    uint16_t type;
>> > +
>> > +#define XENMEM_resource_grant_table 0
>> > +
>> > +    /*
>> > +     * IN - a type-specific resource identifier, which must be zero
>> > +     *      unless stated otherwise.
>> > +     */
>> > +    uint32_t id;
>> > +    /* IN - number of (4K) frames of the resource to be mapped */
>> > +    uint32_t nr_frames;
>> > +    /* IN - the index of the initial frame to be mapped */
>> > +    uint64_aligned_t frame;
>> > +    /* IN/OUT - If the tools domain is PV then, upon return, gmfn_list
>> > +     *          will be populated with the MFNs of the resource.
>> > +     *          If the tools domain is HVM then it is expected that, on
>> > +     *          entry, gmfn_list will be populated with a list of GFNs
>> > +     *          that will be mapped to the MFNs of the resource.
>> > +     */
>> > +    XEN_GUEST_HANDLE(xen_pfn_t) gmfn_list;
>> 
>> Why is it not possible to make PV does the same thing as HVM?
> 
> Because PV guests don't use a P2M as such.

They certainly do, just Xen can't rely on (and hence use) it.

> An HVM guest can pass GFNs in and 
> say 'I want the resource mapped here'. A PV guest can't do that since it's 
> using MFNs directly... it has to deal with the resource wherever it may be.

Xen does, however, maintain the M2P, so it would not be impossible
to return GFNs here for PV guests, requiring the caller to translate
them back to MFNs if so desired.

Jan


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