linux-kernel - Re: [External] Re: [PATCH] iommu: riscv: Split 8-byte accesses on 32 bit I/O bus platform

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Message-ID: <20250401220201.1fa3ec76@pumpkin>
Date: Tue, 1 Apr 2025 22:02:01 +0100
From: David Laight <david.laight.linux@...il.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Xu Lu <luxu.kernel@...edance.com>, Jessica Clarke <jrtc27@...c27.com>,
 tjeznach@...osinc.com, joro@...tes.org, will@...nel.org,
 robin.murphy@....com, alex@...ti.fr, lihangjing@...edance.com,
 xieyongji@...edance.com, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org, iommu@...ts.linux.dev
Subject: Re: [External] Re: [PATCH] iommu: riscv: Split 8-byte accesses on
 32 bit I/O bus platform

On Tue, 1 Apr 2025 12:44:12 -0300
Jason Gunthorpe <jgg@...pe.ca> wrote:

> On Wed, Mar 26, 2025 at 11:26:07AM +0800, Xu Lu wrote:
> > Hi Jessica,
> >   
> > > Is such a platform conformant to the specification?  
> > 
> > We have talked about this before [1]. I think the IOMMU spec does not
> > mandate the implementation of 8-byte access functionality. The related
> > sentences are listed below:
> > 
> > "The 8-byte IOMMU registers are defined in such a way that software
> > can perform two individual 4-byte accesses, or hardware can perform
> > two independent 4-byte transactions resulting from an 8-byte access,
> > to the high and low halves of the register, in that order, as long as
> > the register semantics, with regard to side-effects, are respected
> > between the two software accesses, or two hardware transactions,
> > respectively."  
> 
> I think the commit message should explain an anyalsis that the code is
> safe against the mentioned side effects due to ordering.
> 
> And a comment should explain this:
> 
> +#define riscv_iommu_writeq(iommu, addr, val) \
> +       ((addr == RISCV_IOMMU_REG_IOHPMCYCLES) ? \
> +        lo_hi_writeq_relaxed((val), (iommu)->reg + (addr)) : \
> +        hi_lo_writeq_relaxed((val), (iommu)->reg + (addr)))
> 
> As the naive reading of the above spec paragraph doesn't seem like
> there are exceptions or why one register has to be the opposite order.
> 
> Also missing () around addr

It is also double-evaluating (addr).

I hope there is a lock, interleaved accesses from multiple cpu
may not work.

	David

> 
> Jason
>