Re: [RFC PATCH] mm/vmalloc: add vmalloc_decrypted() and vzalloc_decrypted()

[Jason Gunthorpe <jgg@ziepe.ca>]

On Mon, Jun 08, 2026 at 04:37:02PM +0100, Catalin Marinas wrote:
> > +/**
> > + * vzalloc_decrypted - allocate zeroed virtually contiguous decrypted memory
> > + * @size:    allocation size
> > + *
> > + * Like vmalloc_decrypted(), but the memory is set to zero.
> > + *
> > + * Return: pointer to the allocated memory or %NULL on error
> > + */
> > +void *vzalloc_decrypted_noprof(unsigned long size)
> > +{
> > +	void *addr;
> > +
> > +	addr = __vmalloc_node_range_noprof(size, 1, VMALLOC_START, VMALLOC_END,
> > +					   GFP_KERNEL,
> > +					   pgprot_decrypted(PAGE_KERNEL),
> > +					   VM_DECRYPTED, NUMA_NO_NODE,
> > +					   __builtin_return_address(0));
> > +	if (addr)
> > +		memset(addr, 0, size);
> 
> Talking to Suzuki, the small window between set_memory_decrypted() and
> memset() potentially exposing stale data is safe, at least for Arm CCA
> as the memory would be scrubbed (there are other places in the kernel
> where we do something similar). I assume that's also the case for other
> architectures, although not sure what pKVM does.

It seems like a poor practice though, this should probably be
re-organized to use __GFP_ZERO so things are ordered sensibly.

But what is the purpose of this? I guess some hyperv thing - but
shouldn't we have a more structured way to "DMA map" things for the
hypervisor instead of stuff like this? Why can't you use
dma_alloc_coherent() which actually gives you an address that is
sensible to pass to the hypervisor?

Jason