Michael S. Tsirkin <mst@xxxxxxxxxx> writes: > On Wed, Apr 24, 2019 at 10:01:56PM -0300, Thiago Jung Bauermann wrote: >> >> Michael S. Tsirkin <mst@xxxxxxxxxx> writes: >> >> > On Wed, Apr 17, 2019 at 06:42:00PM -0300, Thiago Jung Bauermann wrote: >> >> >> >> Michael S. Tsirkin <mst@xxxxxxxxxx> writes: >> >> >> >> > On Thu, Mar 21, 2019 at 09:05:04PM -0300, Thiago Jung Bauermann wrote: >> >> >> >> >> >> Michael S. Tsirkin <mst@xxxxxxxxxx> writes: >> >> >> >> >> >> > On Wed, Mar 20, 2019 at 01:13:41PM -0300, Thiago Jung Bauermann wrote: >> >> >> >> >From what I understand of the ACCESS_PLATFORM definition, the host will >> >> >> >> only ever try to access memory addresses that are supplied to it by the >> >> >> >> guest, so all of the secure guest memory that the host cares about is >> >> >> >> accessible: >> >> >> >> >> >> >> >> If this feature bit is set to 0, then the device has same access to >> >> >> >> memory addresses supplied to it as the driver has. In particular, >> >> >> >> the device will always use physical addresses matching addresses >> >> >> >> used by the driver (typically meaning physical addresses used by the >> >> >> >> CPU) and not translated further, and can access any address supplied >> >> >> >> to it by the driver. When clear, this overrides any >> >> >> >> platform-specific description of whether device access is limited or >> >> >> >> translated in any way, e.g. whether an IOMMU may be present. >> >> >> >> >> >> >> >> All of the above is true for POWER guests, whether they are secure >> >> >> >> guests or not. >> >> >> >> >> >> >> >> Or are you saying that a virtio device may want to access memory >> >> >> >> addresses that weren't supplied to it by the driver? >> >> >> > >> >> >> > Your logic would apply to IOMMUs as well. For your mode, there are >> >> >> > specific encrypted memory regions that driver has access to but device >> >> >> > does not. that seems to violate the constraint. >> >> >> >> >> >> Right, if there's a pre-configured 1:1 mapping in the IOMMU such that >> >> >> the device can ignore the IOMMU for all practical purposes I would >> >> >> indeed say that the logic would apply to IOMMUs as well. :-) >> >> >> >> >> >> I guess I'm still struggling with the purpose of signalling to the >> >> >> driver that the host may not have access to memory addresses that it >> >> >> will never try to access. >> >> > >> >> > For example, one of the benefits is to signal to host that driver does >> >> > not expect ability to access all memory. If it does, host can >> >> > fail initialization gracefully. >> >> >> >> But why would the ability to access all memory be necessary or even >> >> useful? When would the host access memory that the driver didn't tell it >> >> to access? >> > >> > When I say all memory I mean even memory not allowed by the IOMMU. >> >> Yes, but why? How is that memory relevant? > > It's relevant when driver is not trusted to only supply correct > addresses. The feature was originally designed to support userspace > drivers within guests. Ah, thanks for clarifying. I don't think that's a problem in our case. If the guest provides an incorrect address, the hardware simply won't allow the host to access it. >> >> >> > Another idea is maybe something like virtio-iommu? >> >> >> >> >> >> You mean, have legacy guests use virtio-iommu to request an IOMMU >> >> >> bypass? If so, it's an interesting idea for new guests but it doesn't >> >> >> help with guests that are out today in the field, which don't have A >> >> >> virtio-iommu driver. >> >> > >> >> > I presume legacy guests don't use encrypted memory so why do we >> >> > worry about them at all? >> >> >> >> They don't use encrypted memory, but a host machine will run a mix of >> >> secure and legacy guests. And since the hypervisor doesn't know whether >> >> a guest will be secure or not at the time it is launched, legacy guests >> >> will have to be launched with the same configuration as secure guests. >> > >> > OK and so I think the issue is that hosts generally fail if they set >> > ACCESS_PLATFORM and guests do not negotiate it. >> > So you can not just set ACCESS_PLATFORM for everyone. >> > Is that the issue here? >> >> Yes, that is one half of the issue. The other is that even if hosts >> didn't fail, existing legacy guests wouldn't "take the initiative" of >> not negotiating ACCESS_PLATFORM to get the improved performance. They'd >> have to be modified to do that. > > So there's a non-encrypted guest, hypervisor wants to set > ACCESS_PLATFORM to allow encrypted guests but that will slow down legacy > guests since their vIOMMU emulation is very slow. Yes. > So enabling support for encryption slows down non-encrypted guests. Not > great but not the end of the world, considering even older guests that > don't support ACCESS_PLATFORM are completely broken and you do not seem > to be too worried by that. Well, I guess that would be the third half of the issue. :-) > For future non-encrypted guests, bypassing the emulated IOMMU for when > that emulated IOMMU is very slow might be solvable in some other way, > e.g. with virtio-iommu. Which reminds me, could you look at > virtio-iommu as a solution for some of the issues? > Review of that patchset from that POV would be appreciated. Yes, I will have a look. As you mentioned already, virtio-iommu doesn't define a way to request iommu bypass for a device so that would have to be added. Though to be honest in practice I don't think such a feature in virtio-iommu would make things easier for us, at least in the short term. It would take the same effort to define a powerpc-specific hypercall to accomplish the same thing (easier, in fact since we wouldn't have to implement the rest of virtio-iommu). In fact, there already is such hypercall, but it is only defined for VIO devices (RTAS_IBM_SET_TCE_BYPASS in QEMU). We would have to make it work on virtio devices as well. -- Thiago Jung Bauermann IBM Linux Technology Center _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
