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

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Date: Thu, 8 Feb 2024 13:03:27 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: ankita@...dia.com
Cc: jgg@...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,
	yi.l.liu@...el.com, ardb@...nel.org, akpm@...ux-foundation.org,
	andreyknvl@...il.com, wangjinchao@...sion.com, gshan@...hat.com,
	ricarkol@...gle.com, linux-mm@...ck.org, lpieralisi@...nel.org,
	rananta@...gle.com, ryan.roberts@....com, aniketa@...dia.com,
	cjia@...dia.com, kwankhede@...dia.com, targupta@...dia.com,
	vsethi@...dia.com, acurrid@...dia.com, apopple@...dia.com,
	jhubbard@...dia.com, danw@...dia.com, kvmarm@...ts.linux.dev,
	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 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).

-- 
Catalin