Re: [RFC v2 28/32] x86/tdx: Make pages shared in ioremap()

[Kuppuswamy, Sathyanarayanan]

On 5/26/21 3:14 PM, Tom Lendacky wrote:
> On 5/26/21 5:02 PM, Tom Lendacky wrote:
> > On 5/26/21 4:37 PM, Kuppuswamy, Sathyanarayanan wrote:
> > > On 5/21/21 9:19 AM, Tom Lendacky wrote:
> > > > In arch/x86/mm/mem_encrypt.c, sme_early_init() (should have renamed that
> > > > when SEV support was added), we do:
> > > >      if (sev_active())
> > > >          swiotlb_force = SWIOTLB_FORCE;
> > > >
> > > > TDX should be able to do a similar thing without having to touch
> > > > arch/x86/kernel/pci-swiotlb.c.
> > > >
> > > > That would remove any confusion over SME being part of a
> > > > protected_guest_has() call.
> > >
> > > You mean sme_active() check in arch/x86/kernel/pci-swiotlb.c is redundant?
> >
> > No, the sme_active() check is required to make sure that SWIOTLB is
> > available under SME. Encrypted DMA is supported under SME if the device
> > supports 64-bit DMA. But if the device doesn't support 64-bit DMA and the
> > IOMMU is not active, then DMA will be bounced through SWIOTLB.
> >
> > As compared to SEV, where all DMA has to be bounced through SWIOTLB or
> > unencrypted memory. For that, swiotlb_force is used.
>
> I should probably add that SME is memory encryption support for
> host/hypervisor/bare-metal, while SEV is memory encryption support for
> virtualization.

Got it. Thanks for clarification.

> Thanks,
> Tom
>
> > Thanks,
> > Tom
> >
> > >   41 int __init pci_swiotlb_detect_4gb(void)
> > >   42 {
> > >   43         /* don't initialize swiotlb if iommu=off (no_iommu=1) */
> > >   44         if (!no_iommu && max_possible_pfn > MAX_DMA32_PFN)
> > >   45                 swiotlb = 1;
> > >   46
> > >   47         /*
> > >   48          * If SME is active then swiotlb will be set to 1 so that bounce
> > >   49          * buffers are allocated and used for devices that do not support
> > >   50          * the addressing range required for the encryption mask.
> > >   51          */
> > >   52         if (sme_active() || is_tdx_guest())
> > >   53                 swiotlb = 1;

--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer