Re: [PATCH v2 13/17] xen/riscv: Implement p2m_entry_from_mfn() and support PBMT configuration

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 7/21/25 2:18 PM, Jan Beulich wrote:

> On 18.07.2025 11:52, Oleksii Kurochko wrote:
> > On 7/17/25 12:25 PM, Jan Beulich wrote:
> > > On 17.07.2025 10:56, Oleksii Kurochko wrote:
> > > > On 7/16/25 6:18 PM, Jan Beulich wrote:
> > > > > On 16.07.2025 18:07, Oleksii Kurochko wrote:
> > > > > > On 7/16/25 1:31 PM, Jan Beulich wrote:
> > > > > > > On 15.07.2025 16:47, Oleksii Kurochko wrote:
> > > > > > > > On 7/1/25 5:08 PM, Jan Beulich wrote:
> > > > > > > > > On 10.06.2025 15:05, Oleksii Kurochko wrote:
> > > > > > > > > > --- a/xen/arch/riscv/p2m.c
> > > > > > > > > > +++ b/xen/arch/riscv/p2m.c
> > > > > > > > > > @@ -345,6 +345,26 @@ static pte_t *p2m_get_root_pointer(struct p2m_domain *p2m, gfn_t gfn)
> > > > > > > > > >          return __map_domain_page(p2m->root + root_table_indx);
> > > > > > > > > >      }
> > > > > > > > > >      
> > > > > > > > > > +static int p2m_type_radix_set(struct p2m_domain *p2m, pte_t pte, p2m_type_t t)
> > > > > > > > > >                       
> > > > > > > > > See comments on the earlier patch regarding naming.
> > > > > > > > >
> > > > > > > > > > +{
> > > > > > > > > > +    int rc;
> > > > > > > > > > +    gfn_t gfn = mfn_to_gfn(p2m->domain, mfn_from_pte(pte));
> > > > > > > > > >                       
> > > > > > > > > How does this work, when you record GFNs only for Xenheap pages?
> > > > > > > > >                     
> > > > > > > > I think I don't understand what is an issue. Could you please provide
> > > > > > > > some extra details?
> > > > > > > >                   
> > > > > > > Counter question: The mfn_to_gfn() you currently have is only a stub. It only
> > > > > > > works for 1:1 mapped domains. Can you show me the eventual final implementation
> > > > > > > of the function, making it possible to use it here?
> > > > > > >                 
> > > > > > At the moment, I planned to support only 1:1 mapped domains, so it is final
> > > > > > implementation.
> > > > > >               
> > > > > Isn't that on overly severe limitation?
> > > > >             
> > > > I wouldn't say that it's a severe limitation, as it's just a matter of how
> > > > |mfn_to_gfn()| is implemented. When non-1:1 mapped domains are supported,
> > > > |mfn_to_gfn()| can be implemented differently, while the code where it’s called
> > > > will likely remain unchanged.
> > > >
> > > > What I meant in my reply is that, for the current state and current limitations,
> > > > this is the final implementation of|mfn_to_gfn()|. But that doesn't mean I don't
> > > > see the value in, or the need for, non-1:1 mapped domains—it's just that this
> > > > limitation simplifies development at the current stage of the RISC-V port.
> > > >           
> > > Simplification is fine in some cases, but not supporting the "normal" way of
> > > domain construction looks like a pretty odd restriction. I'm also curious
> > > how you envision to implement mfn_to_gfn() then, suitable for generic use like
> > > the one here. Imo, current limitation or not, you simply want to avoid use of
> > > that function outside of the special gnttab case.
> > >
> > > > > > > > > In this context (not sure if I asked before): With this use of a radix tree,
> > > > > > > > > how do you intend to bound the amount of memory that a domain can use, by
> > > > > > > > > making Xen insert very many entries?
> > > > > > > > >                     
> > > > > > > > I didn’t think about that. I assumed it would be enough to set the amount of
> > > > > > > > memory a guest domain can use by specifying|xen,domain-p2m-mem-mb| in the DTS,
> > > > > > > > or using some predefined value if|xen,domain-p2m-mem-mb| isn’t explicitly set.
> > > > > > > >                   
> > > > > > > Which would require these allocations to come from that pool.
> > > > > > >                 
> > > > > > Yes, and it is true only for non-hardware domains with the current implementation.
> > > > > >               
> > > > > ???
> > > > >             
> > > > I meant that pool is used now only for non-hardware domains at the moment.
> > > >           
> > > And how does this matter here? The memory required for the radix tree doesn't
> > > come from that pool anyway.
> > >         
> > I thought that is possible to do that somehow, but looking at a code of
> > radix-tree.c it seems like the only one way to allocate memroy for the radix
> > tree isradix_tree_node_alloc() -> xzalloc(struct rcu_node).
> >
> > Then it is needed to introduce radix_tree_node_allocate(domain)
> >       
> That would be a possibility, but you may have seen that less than half a
> year ago we got rid of something along these lines. So it would require
> some pretty good justification to re-introduce.
>
> > or radix tree
> > can't be used at all for mentioned in the previous replies security reason, no?
> >       
> (Very) careful use may still be possible. But the downside of using this
> (potentially long lookup times) would always remain.
>     
Could you please clarify what do you mean here by "(Very) careful"?
I thought about an introduction of an amount of possible keys in radix tree and if this amount
is 0 then stop domain. And it is also unclear what should be a value for this amount.
Probably, you have better idea.

But generally your idea below ...
> > > > > > > > Also, it seems this would just lead to the issue you mentioned earlier: when
> > > > > > > > the memory runs out,|domain_crash()| will be called or PTE will be zapped.
> > > > > > > >                   
> > > > > > > Or one domain exhausting memory would cause another domain to fail. A domain
> > > > > > > impacting just itself may be tolerable. But a domain affecting other domains
> > > > > > > isn't.
> > > > > > >                 
> > > > > > But it seems like this issue could happen in any implementation. It won't happen only
> > > > > > if we will have only pre-populated pool for any domain type (hardware, control, guest
> > > > > > domain) without ability to extend them or allocate extra pages from domheap in runtime.
> > > > > > Otherwise, if extra pages allocation is allowed then we can't really do something
> > > > > > with this issue.
> > > > > >               
> > > > > But that's why I brought this up: You simply have to. Or, as indicated, the
> > > > > moment you mark Xen security-supported on RISC-V, there will be an XSA needed.
> > > > >             
> > > > Why it isn't XSA for other architectures? At least, Arm then should have such
> > > > XSA.
> > > >           
> > > Does Arm use a radix tree for storing types? It uses one for mem-access, but
> > > it's not clear to me whether that's actually a supported feature.
> > >
> > > > I don't understand why x86 won't have the same issue. Memory is the limited
> > > > and shared resource, so if one of the domain will use to much memory then it could
> > > > happen that other domains won't have enough memory for its purpose...
> > > >           
> > > The question is whether allocations are bounded. With this use of a radix tree,
> > > you give domains a way to have Xen allocate pretty much arbitrary amounts of
> > > memory to populate that tree. That unbounded-ness is the problem, not memory
> > > allocations in general.
> > >         
> > Isn't radix tree key bounded to an amount of GFNs given for a domain? We can't have
> > more keys then a max GFN number for a domain. So a potential amount of necessary memory
> > for radix tree is also bounded to an amount of GFNs.
> >       
> To some degree yes, hence why I said "pretty much arbitrary amounts".
> But recall that "amount of GFNs" is a fuzzy term; I think you mean to
> use it to describe the amount of memory pages given to the guest. GFNs
> can be used for other purposes, though. Guests could e.g. grant
> themselves access to their own memory, then map those grants at
> otherwise unused GFNs.
>
> > Anyway, IIUC I just can't use radix tree for p2m types at all, right?
> > If yes, does it make sense to borrow 2 bits from struct page_info->type_info as now it
> > is used 9-bits for count of a frame?
> >       
> struct page_info describes MFNs, when you want to describe GFNs. As you
> mentioned earlier, multiple GFNs can in principle map to the same MFN.
> You would force them to all have the same properties, which would be in
> direct conflict with e.g. the grant P2M types.
>
> Just to mention one possible alternative to using radix trees: You could
> maintain a 2nd set of intermediate "page tables", just that leaf entries
> would hold meta data for the respective GFN. The memory for those "page
> tables" could come from the normal P2M pool (and allocation would thus
> only consume domain-specific resources). Of course in any model like
> this the question of lookup times (as mentioned above) would remain.
>     
...looks like an optimal option.

The only thing I worry about is that it will require some code duplication
(I will think how to re-use the current one code), as for example, when
setting/getting metadata, TLB flushing isn’t needed at all as we aren't
working with with real P2M page tables.
Agree that lookup won't be the best one, but nothing can be done with
such models.

~ Oleksii