Re: [PATCH v3 9/9] x86/P2M: relax permissions of PVH Dom0's MMIO entries

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

On Tue, Sep 21, 2021 at 09:21:11AM +0200, Jan Beulich wrote:
> To become independent of the sequence of mapping operations, permit
> "access" to accumulate for Dom0, noting that there's not going to be an
> introspection agent for it which this might interfere with. While e.g.
> ideally only ROM regions would get mapped with X set, getting there is
> quite a bit of work. Plus the use of p2m_access_* here is abusive in the
> first place.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> v3: Move last in series, for being controversial.
> v2: Split off from original patch. Accumulate all of R, W, and X.
> 
> --- a/xen/arch/x86/mm/p2m.c
> +++ b/xen/arch/x86/mm/p2m.c
> @@ -1319,6 +1319,18 @@ static int set_typed_p2m_entry(struct do
>              return -EPERM;
>          }
>  
> +        /*
> +         * Gross bodge, to go away again rather sooner than later:
> +         *
> +         * For MMIO allow access permissions to accumulate, but only for 
> Dom0.
> +         * Since set_identity_p2m_entry() and set_mmio_p2m_entry() differ in
> +         * the way they specify "access", this will allow the ultimate result
> +         * to be independent of the sequence of operations.

Wouldn't it be better to 'fix' those operations so that they can work
together?

It's my understanding that set_identity_p2m_entry is the one that has
strong requirements regarding the access permissions, as on AMD ACPI
tables can specify how should regions be mapped.

A possible solution might be to make set_mmio_p2m_entry more tolerant
to how present mappings are handled. For once that function doesn't
let callers specify access permissions, so I would consider that if a
mapping is present on the gfn and it already points to the requested
mfn no error should be returned to the caller. At the end the 'default
access' for that gfn -> mfn relation is the one established by
set_identity_p2m_entry and shouldn't be subject to the p2m default
access.

Thanks, Roger.