netdev - Re: [PATCH] vhost: fix caching attributes of MMIO regions by setting them explicitly

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Message-ID: <20260113022538-mutt-send-email-mst@kernel.org>
Date: Tue, 13 Jan 2026 02:30:13 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Kommula Shiva Shankar <kshankar@...vell.com>
Cc: jasowang@...hat.com, virtualization@...ts.linux.dev,
	eperezma@...hat.com, kvm@...r.kernel.org, netdev@...r.kernel.org,
	jerinj@...vell.com, ndabilpuram@...vell.com, schalla@...vell.com,
	dtatulea@...dia.com, jgg@...dia.com
Subject: Re: [PATCH] vhost: fix caching attributes of MMIO regions by setting
 them explicitly

On Fri, Jan 02, 2026 at 12:27:03PM +0530, Kommula Shiva Shankar wrote:
> Explicitly set non-cached caching attributes for MMIO regions.
> Default write-back mode can cause CPU to cache device memory,
> causing invalid reads and unpredictable behavior.
> 
> Invalid read and write issues were observed on ARM64 when mapping the
> notification area to userspace via mmap.

device memory in question is the VQ kick, yes?
So if it is cached, the kick can get delayed, but how
is this causing "invalid read and write issues"?
What is read/written exactly?

> 
> Signed-off-by: Kommula Shiva Shankar <kshankar@...vell.com>
> Acked-by: Jason Wang <jasowang@...hat.com>

I also worry a bit about regressing on other hardware.
Cc nvidia guys.


> ---
> Originally sent to net-next, now redirected to vhost tree
> per Jason Wang's suggestion. 
> 
>  drivers/vhost/vdpa.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> index 05a481e4c385..b0179e8567ab 100644
> --- a/drivers/vhost/vdpa.c
> +++ b/drivers/vhost/vdpa.c
> @@ -1527,6 +1527,7 @@ static int vhost_vdpa_mmap(struct file *file, struct vm_area_struct *vma)
>  	if (vma->vm_end - vma->vm_start != notify.size)
>  		return -ENOTSUPP;
>  
> +	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
>  	vm_flags_set(vma, VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP);
>  	vma->vm_ops = &vhost_vdpa_vm_ops;
>  	return 0;
> -- 
> 2.48.1