Re: [RFC PATCH] arm64: swiotlb: dma: its: Ensure shared buffers are properly aligned

Jason Gunthorpe
2025-09-08T14:58:48Z

On Mon, Sep 08, 2025 at 02:47:21PM +0100, Suzuki K Poulose wrote:
> On 08/09/2025 12:40, Catalin Marinas wrote:
> > On Mon, Sep 08, 2025 at 03:07:00PM +0530, Aneesh Kumar K.V wrote:
> > > Catalin Marinas <catalin.marinas@arm.com> writes:
> > > > On Fri, Sep 05, 2025 at 11:24:41AM +0530, Aneesh Kumar K.V (Arm) wrote:
> > > > > When running with private memory guests, the guest kernel must allocate
> > > > > memory with specific constraints when sharing it with the hypervisor.
> > > > >
> > > > > These shared memory buffers are also accessed by the host kernel, which
> > > > > means they must be aligned to the host kernel's page size.
> > > >
> > > > So this is the case where the guest page size is smaller than the host
> > > > one. Just trying to understand what would go wrong if we don't do
> > > > anything here. Let's say the guest uses 4K pages and the host a 64K
> > > > pages. Within a 64K range, only a 4K is shared/decrypted. If the host
> > > > does not explicitly access the other 60K around the shared 4K, can
> > > > anything still go wrong? Is the hardware ok with speculative loads from
> > > > non-shared ranges?
> > >
> > > With features like guest_memfd, the goal is to explicitly prevent the
> > > host from mapping private memory, rather than relying on the host to
> > > avoid accessing those regions.
> >
> > Yes, if all the memory is private. At some point the guest will start
> > sharing memory with the host. In theory, the host could map more than it
> > was given access to as long as it doesn't touch the area around the
> > shared range. Not ideal and it may not match the current guest_memfd API
>
> The kernel may be taught not to touch the area, but it is tricky when
> the shared page gets mapped into the usespace and what it does with it.

But what happes?

The entire reason we have this nasty hyper-restrictive memfd private
memory is beacuse Intel takes a machine check if anything does it
wrong, and that is fatal and can't be handled.

Is ARM like that? I thought ARM had good faults on GPT violation that
could be handled in the same way as a normal page fault?

If ARM has proper faulting then you don't have an issue mapping 64K
into a userspace and just segfaulting the VMM if it does something
wrong.

If not, then sure you need all this unmapping stuff like Intel does :\

> True. The GPC Page Size is going to be 4K. At present the RMM S2 page
> size is fixed to 4K.

A 4k S2 is a pointless thing to do if the VMM is only going to approve
64k shared/private transitions :(

Jason