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

From: Jason Gunthorpe <jgg@nvidia.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: ankita@nvidia.com, maz@kernel.org, oliver.upton@linux.dev,
	james.morse@arm.com, suzuki.poulose@arm.com,
	yuzenghui@huawei.com, reinette.chatre@intel.com,
	surenb@google.com, stefanha@redhat.com, brauner@kernel.org,
	will@kernel.org, mark.rutland@arm.com,
	alex.williamson@redhat.com, kevin.tian@intel.com,
	yi.l.liu@intel.com, ardb@kernel.org, akpm@linux-foundation.org,
	andreyknvl@gmail.com, wangjinchao@xfusion.com, gshan@redhat.com,
	ricarkol@google.com, linux-mm@kvack.org, lpieralisi@kernel.org,
	rananta@google.com, ryan.roberts@arm.com, aniketa@nvidia.com,
	cjia@nvidia.com, kwankhede@nvidia.com, targupta@nvidia.com,
	vsethi@nvidia.com, acurrid@nvidia.com, apopple@nvidia.com,
	jhubbard@nvidia.com, danw@nvidia.com, kvmarm@lists.linux.dev,
	mochs@nvidia.com, zhiw@nvidia.com, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH v6 2/4] mm: introduce new flag to indicate wc safe
Date: Thu, 8 Feb 2024 10:34:14 -0400	[thread overview]
Message-ID: <20240208143414.GJ10476@nvidia.com> (raw)
In-Reply-To: <ZcTRH1rzfPbuQ_qj@arm.com>

On Thu, Feb 08, 2024 at 01:03:27PM +0000, Catalin Marinas wrote:
> On Thu, Feb 08, 2024 at 02:16:50AM +0530, ankita@nvidia.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@nvidia.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