Re: [PATCH v2 04/17] xen/riscv: construct the P2M pages pool for guests

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 6/18/25 5:53 PM, Jan Beulich wrote:

> On 10.06.2025 15:05, Oleksii Kurochko wrote:
> > @@ -18,10 +20,20 @@ struct arch_vcpu_io {
> >  struct arch_vcpu {
> >  };
> >  
> > +struct paging_domain {
> > +    spinlock_t lock;
> > +    /* Free P2M pages from the pre-allocated P2M pool */
> > +    struct page_list_head p2m_freelist;
> > +    /* Number of pages from the pre-allocated P2M pool */
> > +    unsigned long p2m_total_pages;
> > +};
> > +
> >  struct arch_domain {
> >      struct hvm_domain hvm;
> >  
> >      struct p2m_domain p2m;
> > +
> > +    struct paging_domain paging;
> >       
> With the separate structures, do you have plans to implement e.g. shadow paging?
> Or some other paging mode beyond the basic one based on the H extension?
>     
No, there is no such plans.

>  If the
> structures are to remain separate, may I suggest that you keep things properly
> separated (no matter how e.g. Arm may have it) in terms of naming? I.e. no
> single "p2m" inside struct paging_domain.
>     
Arm doesn't implement shadow paging too (AFAIK) and probably this approach was
copied from x86, and then to RISC-V.
I thought that a reason for that was just to have two separate entities: one which
covers page tables and which covers the full available guest memory.
And if the only idea of that was to have shadow paging then I don't how it should
be done better. As p2m code is based on Arm's, perhaps, it makes sense to have
this stuff separated, so easier porting will be.

> > @@ -105,6 +106,9 @@ int p2m_init(struct domain *d)
> >      struct p2m_domain *p2m = p2m_get_hostp2m(d);
> >      int rc;
> >  
> > +    spin_lock_init(&d->arch.paging.lock);
> > +    INIT_PAGE_LIST_HEAD(&d->arch.paging.p2m_freelist);
> >       
> If you want p2m and paging to be separate, you will want to put these in a new
> paging_init().
>     
I am not really understand what is wrong to have it here, but likely it is because
I don't really get an initial purpose of having p2m and paging separately.
It seems like p2m and paging are connected between each other, so it is fine
to init them together.

~ Oleksii