Re: [PATCH 10/16] xen/arm: add Persistent Map (PMAP) infrastructure

[Julien Grall <julien@xxxxxxx>]

Hi Stefano,

On 08/06/2022 02:08, Stefano Stabellini wrote:
> > diff --git a/xen/arch/arm/include/asm/pmap.h b/xen/arch/arm/include/asm/pmap.h
> > new file mode 100644
> > index 000000000000..74398b4c4fe6
> > --- /dev/null
> > +++ b/xen/arch/arm/include/asm/pmap.h
> > @@ -0,0 +1,32 @@
> > +#ifndef __ASM_PMAP_H__
> > +#define __ASM_PMAP_H__
> > +
> > +#include <xen/mm.h>
> > +
> > +#include <asm/fixmap.h>
> > +
> > +static inline void arch_pmap_map(unsigned int slot, mfn_t mfn)
> > +{
> > +    lpae_t *entry = &xen_fixmap[slot];
> > +    lpae_t pte;
> > +
> > +    ASSERT(!lpae_is_valid(*entry));
> > +
> > +    pte = mfn_to_xen_entry(mfn, PAGE_HYPERVISOR_RW);
> > +    pte.pt.table = 1;
> > +    write_pte(entry, pte);
> Here we don't need a tlb flush because we never go from a valid mapping
> to another valid mapping.
A TLB flush would not be sufficient here. You would need to follow the 
break-before-make sequence in order to replace a valid mapping with 
another valid mapping.
> We also go through arch_pmap_unmap which
> clears the mapping and also flushes the tlb. Is that right?
The PMAP code is using a bitmap to know which entry is used. So when 
arch_pmap_map() is called, we also guarantees the entry will be invalid 
(hence the ASSERT(!lpae_is_valid()).
The bit in the bitmap will only be cleared by pmap_unmap() which will 
result to a TLB flush.
> > +}
> > +
> > +static inline void arch_pmap_unmap(unsigned int slot)
> > +{
> > +    lpae_t pte = {};
> > +
> > +    write_pte(&xen_fixmap[slot], pte);
> > +
> > +    flush_xen_tlb_range_va_local(FIXMAP_ADDR(slot), PAGE_SIZE);
> > +}
> > +
> > +void arch_pmap_map_slot(unsigned int slot, mfn_t mfn);
> > +void arch_pmap_clear_slot(void *ptr);
> What are these two? They are not defined anywhere?
It is left-over. I will drop them.

Cheers,

--
Julien Grall