linux-kernel - Re: [PATCH v6 2/4] mm: introduce new flag to indicate wc safe

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Message-ID: <20240208143414.GJ10476@nvidia.com>
Date: Thu, 8 Feb 2024 10:34:14 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: ankita@...dia.com, maz@...nel.org, oliver.upton@...ux.dev,
	james.morse@....com, suzuki.poulose@....com, yuzenghui@...wei.com,
	reinette.chatre@...el.com, surenb@...gle.com, stefanha@...hat.com,
	brauner@...nel.org, will@...nel.org, mark.rutland@....com,
	alex.williamson@...hat.com, kevin.tian@...el.com,
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	mochs@...dia.com, zhiw@...dia.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v6 2/4] mm: introduce new flag to indicate wc safe

On Thu, Feb 08, 2024 at 01:03:27PM +0000, Catalin Marinas wrote:
> On Thu, Feb 08, 2024 at 02:16:50AM +0530, ankita@...dia.com wrote:
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index f5a97dec5169..884c068a79eb 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -391,6 +391,20 @@ extern unsigned int kobjsize(const void *objp);
> >  # define VM_UFFD_MINOR		VM_NONE
> >  #endif /* CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */
> >  
> > +/*
> > + * This flag is used to connect VFIO to arch specific KVM code. It
> > + * indicates that the memory under this VMA is safe for use with any
> > + * non-cachable memory type inside KVM. Some VFIO devices, on some
> > + * platforms, are thought to be unsafe and can cause machine crashes if
> > + * KVM does not lock down the memory type.
> > + */
> > +#ifdef CONFIG_64BIT
> > +#define VM_VFIO_ALLOW_WC_BIT	39
> > +#define VM_VFIO_ALLOW_WC	BIT(VM_VFIO_ALLOW_WC_BIT)
> > +#else
> > +#define VM_VFIO_ALLOW_WC	VM_NONE
> > +#endif
> 
> Adding David Hildenbrand to this thread as well since we briefly
> discussed potential alternatives (not sure we came to any conclusion).

FWIW, with my mm hat on:

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

But I'm interested if David has an alternative. We don't have a
shortage of bits here so I'm not sure it is worth much fuss.

Jason