Re: Why doesn't arm-smmu (v2) driver take VA_BITS into consideration?

From: Nicolin Chen <nicoleotsuka@gmail.com>
To: Will Deacon <will@kernel.org>
Cc: will.deacon@arm.com, robin.murphy@arm.com,
	linux-arm-kernel@lists.infradead.org
Subject: Re: Why doesn't arm-smmu (v2) driver take VA_BITS into consideration?
Date: Fri, 28 Jun 2019 01:10:07 -0700	[thread overview]
Message-ID: <20190628081006.GA6628@Asurada> (raw)
In-Reply-To: <20190627102640.2pzgiro3gq3ont5s@willie-the-truck>

Hi Will,

On Thu, Jun 27, 2019 at 11:26:40AM +0100, Will Deacon wrote:
> On Wed, Jun 26, 2019 at 05:19:33PM -0700, Nicolin Chen wrote:
> > I am using an Soc that contains arm-smmu v2 engines. I found that
> > the arm-smmu driver reads GR_ID2 register and defines ias and oas
> > of each domain based on the corresponding fields of that GR_ID2.
> > 
> > Using my platform for example, it gets 48-bit for all ias and oas,
> > being translated from those fields so I got these 2 stages:
> >         Stage-1: 48-bit VA -> 48-bit IPA
> >         Stage-2: 48-bit IPA -> 48-bit PA
> > 
> > Then the code does below to configure aperture_end to 48-bit:
> >  914         domain->geometry.aperture_end = (1UL << ias) - 1;
> > 
> > However, my system configures VA_BITS to 39. So aperture_end==48
> > is a mismatch comparing to VA_BITS_39. Although the aperture_end
> > is supposed to cap IOVA allocation, this 48-bit cap won't really
> > do the job at all.
> 
> What actually goes wrong, though? IOVAs should only be handed over to
> devices, so VA_BITS doesn't strictly matter. It's the DMA masks that are
> important.

Ah..that's true. I think I misunderstood one of my situations here.

> > I saw that arm-smmu-v3 driver takes VA_BITS into consideration:
> >  1765         switch (smmu_domain->stage) {
> >  1766         case ARM_SMMU_DOMAIN_S1:
> >  1767                 ias = (smmu->features & ARM_SMMU_FEAT_VAX) ?
> >  1768                 ias = min_t(unsigned long, ias, VA_BITS);
> > 
> > So I am wondering if it is intentionally designed so by ignoring
> > VA_BITS? Would you please help me understand this a bit?
> 
> I think it's simply that SMMUv3 doesn't provide an ID field to tell you
> what to use, so we chose to go with VA_BITS since it's what we're using
> on the CPU.

I see. Thanks for explaining!

Nicolin

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel