Re: [PATCHv8 00/11] Linear Address Masking enabling

[Jacob Pan]

Hi Jason,

On Tue, 20 Sep 2022 15:50:33 -0300, Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:

> On Tue, Sep 20, 2022 at 11:41:04AM -0700, Jacob Pan wrote:
> > Hi Jason,
> > 
> > On Tue, 20 Sep 2022 13:27:27 -0300, Jason Gunthorpe <jgg@xxxxxxxxxx>
> > wrote: 
> > > On Tue, Sep 20, 2022 at 09:06:32AM -0700, Dave Hansen wrote:  
> > > > On 9/20/22 06:14, Jason Gunthorpe wrote:    
> > > > > For this I would rather have a function that queries the format
> > > > > of the page table under the mm_struct and we have enum values like
> > > > > INTEL_NORMAL and INTEL_LAM as possible values.
> > > > > 
> > > > > The iommu driver will block incompatible page table formats, and
> > > > > when it starts up it should assert something that blocks changing
> > > > > the format.    
> > > > 
> > > > That doesn't sound too bad.  Except, please don't call it a "page
> > > > table format".  The format of the page tables does not change with
> > > > LAM.  It's entirely how the CPU interprets addresses that changes.
> > > >   
> > > 
> > > Sure it does. The rules for how the page table is walked change. The
> > > actual bits stored in memory might not be different, but that doesn't
> > > mean the format didn't change. If it didn't change we wouldn't have an
> > > incompatibility with the IOMMU HW walker.  
> > 
> > There are many CPU-IOMMU compatibility checks before we do for SVA,e.g.
> > we check paging mode in sva_bind. We are delegating these checks in
> > arch/platform code. So why can't we let arch code decide how to convey
> > mm-IOMMU SVA compatibility? let it be a flag ( as in this patch) or some
> > callback.  
> 
> In general I'm not so keen on arch unique code for general ideas like
> this (ARM probably has the same issue), but sure it could work.
> 
Creating an abstraction seems to belong to a separate endeavor when we
have more than one user. For now, are you ok with  the current approach?

> > Perhaps a more descriptive name
> > s/arch_can_alloc_pasid(mm)/arch_can_support_sva(mm)/ is all we
> > disagreeing :)  
> 
> Except that still isn't what it is doing. "sva" can mean lots of
> things.
True, but sharing page table is the root cause of the incompatibility.
IMHO, SVA means sharing page table  at the highest level.

> You need to assert that the page table format is one of the
> formats that the iommu understands and configure the iommu to match
> it. It is a very simple question about what ruleset and memory layout
> govern the page table memory used by the CPU.
the problem is more relevant to  things like  canonical address
requirement  than  page table format.

> And I think every CPU should be able to define a couple of their
> configurations in some enum, most of the PTE handling code is all
> hardwired, so I don't think we really support that many combinations
> anyhow?
sounds like a nice but separate effort,  I don't know enough here.


Thanks,

Jacob