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.

Thanks, Roger.

Roger Pau Monne (2):
  xen/x86: account for max guest gfn and number of foreign mappings in
    the p2m
  xen/x86: remove foreign mappings from the p2m on teardown

 CHANGELOG.md                   |  1 +
 xen/arch/x86/domain.c          |  6 +++
 xen/arch/x86/include/asm/p2m.h | 28 ++++++++++----
 xen/arch/x86/mm/p2m.c          | 70 ++++++++++++++++++++++++++++++++--
 4 files changed, 94 insertions(+), 11 deletions(-)

-- 
2.44.0

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.

Hi Roger,

On Tue, 2024-04-30 at 18:58 +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.
In case of it will be properly tested I think we can consider this
patch as a candidate for 4.19

~ Oleksii

> 
> Thanks, Roger.
> 
> Roger Pau Monne (2):
>   xen/x86: account for max guest gfn and number of foreign mappings
> in
>     the p2m
>   xen/x86: remove foreign mappings from the p2m on teardown
> 
>  CHANGELOG.md                   |  1 +
>  xen/arch/x86/domain.c          |  6 +++
>  xen/arch/x86/include/asm/p2m.h | 28 ++++++++++----
>  xen/arch/x86/mm/p2m.c          | 70
> ++++++++++++++++++++++++++++++++--
>  4 files changed, 94 insertions(+), 11 deletions(-)
>