Re: [PATCH v2 5/8] xen/riscv: introduce asm/pmap.h header

[oleksii.kurochko@xxxxxxxxx]

On Tue, 2024-07-23 at 10:36 +0200, Jan Beulich wrote:
> On 23.07.2024 10:02, Oleksii Kurochko wrote:
> > On Mon, Jul 22, 2024 at 7:27 PM Julien Grall <julien@xxxxxxx>
> > wrote:
> > > > > On 22/07/2024 15:44, Oleksii Kurochko wrote:
> > > >     /* Map a 4k page in a fixmap entry */
> > > >     void set_fixmap(unsigned map, mfn_t mfn, unsigned int
> > > > flags)
> > > >     {
> > > >         pte_t pte;
> > > > 
> > > >         pte = mfn_to_xen_entry(mfn, flags);
> > > >         pte.pte |= PTE_LEAF_DEFAULT;
> > > >         write_pte(&xen_fixmap[pt_index(0, FIXMAP_ADDR(map))],
> > > > pte);
> > > 
> > > It would be saner to check if you are not overwriting any
> > > existing
> > > mapping as otherwise you will probably need a TLB flush.
> > > 
> > > >     }
> > > > 
> > > >     /* Remove a mapping from a fixmap entry */
> > > >     void clear_fixmap(unsigned map)
> > > >     {
> > > >         pte_t pte = {0};
> > > >         write_pte(&xen_fixmap[pt_index(0, FIXMAP_ADDR(map))],
> > > > pte);
> > > 
> > > Don't you need a TLB flush?
> > > 
> > Inside write_pte() there is "sfence.vma".
> 
> That's just a fence though, not a TLB flush.
>From the privileged doc:
   ```
   SFENCE.VMA is also used to invalidate entries in the
   address-translation cache associated with a hart (see Section 4.3.2). 
   ...
   The SFENCE.VMA is used to flush any local hardware caches related to
   address translation.
   It is specified as a fence rather than a TLB flush to provide cleaner
   semantics with respect to
   which instructions are affected by the flush operation and to support a
   wider variety of dynamic
   caching structures and memory-management schemes. SFENCE.VMA is also
   used by higher
   privilege levels to synchronize page table writes and the address
   translation hardware.
   ...
   ```
I read this as SFENCE.VMA is used not only for ordering of load/stores,
but also to flush TLB ( which is a type of more general term as
address-translation cache, IIUIC ).

Also, Linux kernel uses sfence.vma to flush TLB:
https://elixir.bootlin.com/linux/v6.0/source/arch/riscv/include/asm/tlbflush.h#L23

~ Oleksii