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

From: Stefano Stabellini <sstabellini@kernel.org>
To: Julien Grall <julien@xen.org>
Cc: "Stefano Stabellini" <sstabellini@kernel.org>,
	xen-devel@lists.xenproject.org, "Wei Liu" <wei.liu2@citrix.com>,
	"Bertrand Marquis" <bertrand.marquis@arm.com>,
	"Volodymyr Babchuk" <Volodymyr_Babchuk@epam.com>,
	"George Dunlap" <george.dunlap@citrix.com>,
	"Hongyan Xia" <hongyxia@amazon.com>,
	"Julien Grall" <jgrall@amazon.com>,
	"Jan Beulich" <jbeulich@suse.com>, "Wei Liu" <wl@xen.org>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [PATCH 10/16] xen/arm: add Persistent Map (PMAP) infrastructure
Date: Wed, 8 Jun 2022 17:25:24 -0700 (PDT)	[thread overview]
Message-ID: <alpine.DEB.2.22.394.2206081725040.21215@ubuntu-linux-20-04-desktop> (raw)
In-Reply-To: <dec50384-5172-67b6-f4ac-a9c40d80a641@xen.org>

On Wed, 8 Jun 2022, Julien Grall wrote:
> 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.

With those two functions removed, you can add my 

Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>