Re: [PATCH v2 03/10] xen/arm: smmuv3: Ensure queue is read after updating prod pointer

[Julien Grall <julien@xxxxxxx>]

On 05/09/2022 10:18, Rahul Singh wrote:
> > On 3 Sep 2022, at 12:21 am, Stefano Stabellini <sstabellini@xxxxxxxxxx> wrote:
> >
> > On Fri, 2 Sep 2022, Rahul Singh wrote:
> > > From: Zhou Wang <wangzhou1@xxxxxxxxxxxxx>
> > >
> > > Backport Linux commit a76a37777f2c. This is the clean backport without
> > > any changes.
> > >
> > > Reading the 'prod' MMIO register in order to determine whether or
> > > not there is valid data beyond 'cons' for a given queue does not
> > > provide sufficient dependency ordering, as the resulting access is
> > > address dependent only on 'cons' and can therefore be speculated
> > > ahead of time, potentially allowing stale data to be read by the
> > > CPU.
> > >
> > > Use readl() instead of readl_relaxed() when updating the shadow copy
> > > of the 'prod' pointer, so that all speculated memory reads from the
> > > corresponding queue can occur only from valid slots.
> > >
> > > Signed-off-by: Zhou Wang <wangzhou1@xxxxxxxxxxxxx>
> > > Link: 
> > > https://lore.kernel.org/r/1601281922-117296-1-git-send-email-wangzhou1@xxxxxxxxxxxxx
> > > [will: Use readl() instead of explicit barrier. Update 'cons' side to match.]
> > > Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> > > Origin: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 
> > > a76a37777f2c
> > > Signed-off-by: Rahul Singh <rahul.singh@xxxxxxx>
> > > ---
> > > Changes in v2:
> > > - fix commit msg
> > > - add _iomb changes also from the origin patch
> > > ---
> > > xen/arch/arm/include/asm/system.h     |  1 +
> > > xen/drivers/passthrough/arm/smmu-v3.c | 11 +++++++++--
> > > 2 files changed, 10 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/xen/arch/arm/include/asm/system.h 
> > > b/xen/arch/arm/include/asm/system.h
> > > index 65d5c8e423..fe27cf8c5e 100644
> > > --- a/xen/arch/arm/include/asm/system.h
> > > +++ b/xen/arch/arm/include/asm/system.h
> > > @@ -29,6 +29,7 @@
> > > #endif
> > >
> > > #define smp_wmb()       dmb(ishst)
> > > +#define __iomb()        dmb(osh)
> > We don't have any other #define starting with __ in system.h.
> > I wonder if we should call this macro differently or simply iomb().
>   
> I think either iomb() or dma_mb() will be the right name.
> Please let me know your view on this.
It is not 100% clear why Linux went with __iomb() rather than iomb(). 
But I would prefer to keep the __* version to match Linux.
If the others really want to drop the __. Then I think it should be name 
iomb(). The rationale is while __iomb() is an alias to dma_mb(), the 
__iormb() behaves differently compare to dma_mb() (I haven't into 
details why).
So if it was a read barrier, we would likely want to use the iormb() 
semantic. This will keep the terminology consistent with Linux (even if 
we remove the __).
Cheers,

--
Julien Grall