Re: [PATCH for-4.19? 0/2] xen/x86: support foreign mappings for HVM

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Apr 30, 2024 at 06:58:43PM +0200, Roger Pau Monne wrote:
> Hello,
> 
> The following series attempts to solve a shortcoming of HVM/PVH guests
> with the lack of support for foreign mappings.  Such lack of support
> prevents using PVH based guests as stubdomains for example.
> 
> Add support in a way similar to how it was done on Arm, by iterating
> over the p2m based on the maximum gfn.
> 
> Mostly untested, sending early in case it could be considered for 4.19.

I've now tested this using the following dummy XTF test:

void test_main(void)
{
    unsigned long idxs = 0;
    xen_pfn_t gpfns = 0;
    int errs = 0, error;
    struct xen_add_to_physmap_batch add = {
        .domid = DOMID_SELF,
        .space = XENMAPSPACE_gmfn_foreign,
        .u.foreign_domid = 0,
        .size = 1,
        .idxs.p = &idxs,
        .gpfns.p = &gpfns,
        .errs.p = &errs,
    };

    error = hypercall_memory_op(XENMEM_add_to_physmap_batch, &add);
    if ( error || errs )
        xtf_error("add_to_physmap_batch failed: %d (errs: %d)\n", error, errs);

    while ( 1 );

    xtf_success(NULL);
}

And avoiding some of the permissions checks in Xen so that an
arbitrary domU could map dom0 gfn 0.  I see count_info increasing by 1
until the XTF guest is killed, at which point the count_info goes back
to the value previous to the map.

Regards, Roger.