From: Dongli Zhang <dongli.zhang@xxxxxxxxxx> Sent: Friday, March 4, 2022 10:28 AM > > Hi Michael, > > On 3/4/22 10:12 AM, Michael Kelley (LINUX) wrote: > > From: Christoph Hellwig <hch@xxxxxx> Sent: Tuesday, March 1, 2022 2:53 AM > >> > >> Power SVM wants to allocate a swiotlb buffer that is not restricted to low memory for > >> the trusted hypervisor scheme. Consolidate the support for this into the swiotlb_init > >> interface by adding a new flag. > > > > Hyper-V Isolated VMs want to do the same thing of not restricting the swiotlb > > buffer to low memory. That's what Tianyu Lan's patch set[1] is proposing. > > Hyper-V synthetic devices have no DMA addressing limitations, and the > > likelihood of using a PCI pass-thru device with addressing limitations in an > > Isolated VM seems vanishingly small. > > > > So could use of the SWIOTLB_ANY flag be generalized? Let Hyper-V init > > code set the flag before swiotlb_init() is called. Or provide a CONFIG > > variable that Hyper-V Isolated VMs could set. > > I used to send 64-bit swiotlb, while at that time people thought it was the same > as Restricted DMA patchset. > > https://lore.kernel.org/all/20210203233709.19819-1-dongli.zhang@xxxxxxxxxx/ > > However, I do not think Restricted DMA patchset is going to supports 64-bit (or > high memory) DMA. Is this what you are looking for? Yes, it looks like your patchset would do what we want for Hyper-V Isolated VMs, but it is a more complex solution than is needed. My assertion is that in some environments, such as Hyper-V Isolated VMs, we're willing to assume all devices are 64-bit DMA capable, and to stop carrying the legacy baggage. Bounce buffering is used for a different scenario (memory encryption), and the bounce buffers can be allocated in high memory. There's no need for a 2nd swiotlb buffer. Michael
