Re: [RFC PATCH V2] xen/gnttab: Store frame GFN in struct page_info on Arm

[Oleksandr <olekstysh@xxxxxxxxx>]

On 15.09.21 13:06, Jan Beulich wrote:

Hi Jan

> On 14.09.2021 22:44, Oleksandr Tyshchenko wrote:
> > --- a/xen/arch/arm/mm.c
> > +++ b/xen/arch/arm/mm.c
> > @@ -1376,14 +1376,18 @@ unsigned long domain_get_maximum_gpfn(struct domain *d)
> >   void share_xen_page_with_guest(struct page_info *page, struct domain *d,
> >                                  enum XENSHARE_flags flags)
> >   {
> > +    unsigned long type_info;
> > +
> >       if ( page_get_owner(page) == d )
> >           return;
> >   
> >       spin_lock(&d->page_alloc_lock);
> >   
> >       /* The incremented type count pins as writable or read-only. */
> > -    page->u.inuse.type_info =
> > -        (flags == SHARE_ro ? PGT_none : PGT_writable_page) | 1;
> > +    type_info = page->u.inuse.type_info & ~(PGT_type_mask | PGT_count_mask);
> > +    page->u.inuse.type_info = type_info |
> > +        (flags == SHARE_ro ? PGT_none : PGT_writable_page) |
> > +        (1UL << PGT_count_base);
> Just as a note: If this was x86 code, I'd request the redundant
> PGT_count_base to be dropped. Constructs like the above is what we
> have MASK_INSR() for.
I got it, I will look at it.


> > --- a/xen/common/page_alloc.c
> > +++ b/xen/common/page_alloc.c
> > @@ -2204,7 +2204,10 @@ void *alloc_xenheap_pages(unsigned int order, unsigned 
> > int memflags)
> >           return NULL;
> >   
> >       for ( i = 0; i < (1u << order); i++ )
> > +    {
> >           pg[i].count_info |= PGC_xen_heap;
> > +        arch_alloc_xenheap_page(&pg[i]);
> > +    }
> >   
> >       return page_to_virt(pg);
> >   }
> > @@ -2222,7 +2225,10 @@ void free_xenheap_pages(void *v, unsigned int order)
> >       pg = virt_to_page(v);
> >   
> >       for ( i = 0; i < (1u << order); i++ )
> > +    {
> >           pg[i].count_info &= ~PGC_xen_heap;
> > +        arch_free_xenheap_page(&pg[i]);
> > +    }
> >   
> >       free_heap_pages(pg, order, true);
> >   }
> You look to only be adjusting the !SEPARATE_XENHEAP instances of the
> functions. Isn't 32-bit Arm using SEPARATE_XENHEAP?
Hmm, looks like yes, thank you. At least config.h defines that on Arm32. 
I will update the instances.

> > --- a/xen/include/asm-arm/mm.h
> > +++ b/xen/include/asm-arm/mm.h
> > @@ -98,9 +98,18 @@ struct page_info
> >   #define PGT_writable_page PG_mask(1, 1)  /* has writable mappings?         */
> >   #define PGT_type_mask     PG_mask(1, 1)  /* Bits 31 or 63.                 */
> >   
> > - /* Count of uses of this frame as its current type. */
> > -#define PGT_count_width   PG_shift(2)
> > -#define PGT_count_mask    ((1UL<<PGT_count_width)-1)
> > + /* 3-bit count of uses of this frame as its current type. */
> > +#define PGT_count_base    PG_shift(4)
> > +#define PGT_count_mask    PG_mask(7, 4)
> > +
> > +/*
> > + * Stored in bits [27:0] or [59:0] GFN if page is used for grant table frame.
> I don't know enough Arm details to tell whether this is properly
> one bit more than the maximum number of physical address bits.
> Without the extra bit you wouldn't be able to tell apart a
> guest specified GFN matching the value of PGT_INVALID_FRAME_GFN
> from an entry which was set from INVALID_GFN.
Really good point.

1. On Arm64 the p2m_ipa_bits could (theoretically) be 64-bit which, I 
assume, corresponds to the maximum guest physical address (1 << 64) - 1 
= 0xFFFFFFFFFFFFFFFF.
To store that GFN we need 52-bit. But, we provide 60-bit field which is 
more than enough, I think. Practically, the maximum supported 
p2m_ipa_bits is 48-bit, so the maximum supported GFN will occupy 36-bit 
only. Everything is ok here.
2. On Arm32 the p2m_ipa_bits is 40-bit which, I assume, corresponds to 
the maximum guest physical address (1 << 40) - 1 = 0xFFFFFFFFFF. To 
store that GFN we need 28-bit. If I did the calculation correctly, what 
we have on Arm32 is that PGT_INVALID_FRAME_GFN == maximum guest physical 
address and it looks like we need and extra bit on Arm32. Do you think 
we need to borrow one more bit from the count portion to stay on the 
safe side?

> > + * This only valid for the xenheap pages.
> > + */
> > +#define PGT_gfn_width     PG_shift(4)
> > +#define PGT_gfn_mask      ((1UL<<PGT_gfn_width)-1)
> Any reason you don't use PG_mask() here? Any open-coding is prone
> to people later making mistakes.
I failed to come up with idea how to do that without #ifdef. As GFN 
starts at bit 0 different first parameter would be needed for PG_mask on 
32-bit and 64-bit systems.
I wonder whether PGC_count_mask/PGT_count_mask are open-coded on Arm/x86 
because of the same reason.

--
Regards,

Oleksandr Tyshchenko