Re: [PATCH v3 3/6] iommu: add ARM short descriptor page table allocator.

From: Yong Wu <yong.wu@mediatek.com>
To: Will Deacon <will.deacon@arm.com>
Cc: Robin Murphy <Robin.Murphy@arm.com>,
	Joerg Roedel <joro@8bytes.org>,
	Thierry Reding <treding@nvidia.com>,
	Mark Rutland <Mark.Rutland@arm.com>,
	Matthias Brugger <matthias.bgg@gmail.com>,
	Daniel Kurtz <djkurtz@google.com>, Tomasz Figa <tfiga@google.com>,
	Lucas Stach <l.stach@pengutronix.de>,
	Rob Herring <robh+dt@kernel.org>,
	Catalin Marinas <Catalin.Marinas@arm.com>,
	"linux-mediatek@lists.infradead.org" 
	<linux-mediatek@lists.infradead.org>,
	Sasha Hauer <kernel@pengutronix.de>,
	"srv_heupstream@mediatek.com" <srv_heupstream@mediatek.com>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	"pebolle@tiscali.nl" <pebolle@tiscali.nl>,
	"arnd@arndb.de" <arnd@arndb.de>,
	"mitchelh@codeaurora.org" <mitchelh@codeaurora.org>,
	"cloud.chou@mediatek.com" <cloud.chou@mediatek.com>,
	"frederic.chen@mediatek.com" <frederic.chen@mediatek.com>
Subject: Re: [PATCH v3 3/6] iommu: add ARM short descriptor page table allocator.
Date: Tue, 28 Jul 2015 21:37:43 +0800	[thread overview]
Message-ID: <1438090663.25925.75.camel@mhfsdcap03> (raw)
In-Reply-To: <20150728110023.GH29209@arm.com>

On Tue, 2015-07-28 at 12:00 +0100, Will Deacon wrote:
> On Tue, Jul 28, 2015 at 06:08:14AM +0100, Yong Wu wrote:
> > On Mon, 2015-07-27 at 15:11 +0100, Will Deacon wrote:
> > > On Mon, Jul 27, 2015 at 03:05:38PM +0100, Robin Murphy wrote:
> > > > On 27/07/15 05:21, Yong Wu wrote:
> > > > >>>>> +       } else {        /* page or largepage */
> > > > >>>>> +               if (quirk & IO_PGTABLE_QUIRK_SHORT_MTK) {
> > > > >>>>> +                       if (large) { /* special Bit */
> > > > >>>>
> > > > >>>> This definitely needs a better comment! What exactly are you doing here
> > > > >>>> and what is that quirk all about?
> > > > >>>
> > > > >>> I use this quirk is for MTK Special Bit as we don't have the XN bit in
> > > > >>> pagetable.
> > > > >>
> > > > >> I'm still not really clear about what this is.
> > > > >
> > > > > There is some difference between the standard spec and MTK HW,
> > > > > Our hw don't implement some bits, like XN and AP.
> > > > > So I add a quirk for MTK special.
> > > > 
> > > > When you say it doesn't implement these bits, do you mean that having 
> > > > them set will lead to Bad Things happening in the hardware, or that it 
> > > > will simply ignore them and not enforce any of the protections they 
> > > > imply? The former case would definitely want clearly documenting 
> > > > somewhere, whereas for the latter case I'm not sure it's even worth the 
> > > > complication of having a quirk - if the value doesn't matter there seems 
> > > > little point in doing a special dance just for the sake of semantic 
> > > > correctness of the in-memory PTEs, in my opinion.
> > > 
> > > Agreed. We should only use quirks if the current (architecturally
> > > compliant) code causes real issues with the hardware. Then the quirk can
> > > be used to either avoid the problematic routines or to take extra steps
> > > to make things work as the architecture intended.
> > > 
> > > I've asked how this IOMMU differs from the architecture on a number of
> > > occasions, but I'm still yet to receive a response other than "it's special".
> > > 
> > 
> > After check further with DE, Our pagetable is refer to ARM-v7's
> > short-descriptor which is a little different from ARM-v8. like bit0(PXN)
> > in section and supersection, I didn't read ARM-v7 spec before, so I add
> > a MTK quirk to disable PXN bit in section and supersection.(if the PXN
> > bit is wrote in ARM-v7 spec, HW will page fault.)
> 
> I've been reviewing this using the ARMv7 ARM (Rev.C of DDI0406C) the whole
> time. PXN is there as an optional field in non-LPAE implementations. That's
> fine and doesn't require any quirks.

Thanks for your confirm.
Then I delete all the PXN bit in here?

Take a example, 
#define ARM_SHORT_PGD_SECTION_XN		(BIT(0) | BIT(4))
I will change it to "BIT(4)".

> 
> > Then I write this code according to ARM-v8 spec defaultly, and add a
> > ARM-v7 quirk?
> 
> No, I don't think you need this, as the v8 and v7 short-descriptor formats
> look compatible to me. You should only need a quirk if architecturally
> compliant code cannot work on your hardware.
> 
> > And there is a little different between ARM-v7 spec and MTK pagetable.
> > It's the XN(bit0) in small page. MTK don't implement XN bit. 
> > The bit[1:0] in MTK's small page should be 2'b10, if it's 2'b11, HW will
> > page fault.
> 
> Aha, thanks! *That* is worthy of a quirk. Something like:
> 
>   IO_PGTABLE_QUIRK_ARM_NO_XN

Thanks, I will add it.

> 
> > (MTK don't implement AP bits too, but HW don't use them, it is ok even
> > though AP bits is wrote)
> 
> Yeah, I think that's fine. The pgtable code will honour the request but
> the h/w will ignore it.
> 
> > In the end, I will add two quirk like this, is it OK?
> 
> I think you only need the one I mentioned above. I don't see the need
> for PXN at all (as I said in the last review).
> 
> Will