Re: AMD SME encrpytion and PCI BAR pages to user space

[Jason Gunthorpe]

On Wed, Oct 21, 2020 at 10:30:23AM -0500, Tom Lendacky wrote:
> On 10/21/20 6:59 AM, Jason Gunthorpe wrote:
> > On Mon, Oct 19, 2020 at 11:36:16AM -0500, Tom Lendacky wrote:
> > 
> >>> io_remap_pfn_range()? Is there use cases where a caller actually wants
> >>> encrypted io memory?
> >>
> >> As long as you never have physical memory / ram being mapped in this path,
> >> it seems that applying pgprot_decrypted() would be ok.
> > 
> > I made a patch along these lines:
> > 
> > https://github.com/jgunthorpe/linux/commit/fc990842983f3530b72fcceafed84bd6075174a1
> > 
> > Just waiting for the 0-day bots to check it
> > 
> > I now have a report that SME works OK but when the same test is done
> > inside a VM with SEV it fails again - is there something else needed
> > for the SEV case?
> 
> Probably. I would assume that it is getting past the MMIO issue, since the
> above patch should cover SEV, too. But, with SEV, all DMA to and from the
> guest is unencrypted. I'm not familiar with how the DMA is setup and
> performed in this situation, but if the DMA is occurring to userspace
> buffers that are mapped as encrypted, then the resulting access will be
> ciphertext (either reading unencrypted data from the device as encrypted
> or writing encrypted data to the device that should be unencrypted). There
> isn't currently an API to allow userspace to change its mapping from
> encrypted to unencrypted.

Oh, interesting.. Yes the issue is no userspace DMA stuff uses the DMA
API correctly (because it is in userspace)

So SWIOTLB tricks don't work, I wish the dma_map could fail for these
situations

I would have guessed it used some vIOMMU and setup decrpytion just
like the host does..

Thanks,
Jason