Re: [PATCH v1 2/2] KVM: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory

Jason Gunthorpe
2023-11-10T14:27:31Z

On Thu, Nov 09, 2023 at 04:34:10PM +0100, Lorenzo Pieralisi wrote:

> Relaxing S2 KVM device MMIO mappings to Normal-NC is not expected to
> trigger any issue on guest device reclaim use cases either (ie device
> MMIO unmap followed by a device reset) at least for PCIe devices, in that
> in PCIe a device reset is architected and carried out through PCI config
> space transactions that are naturally ordered wrt MMIO transactions
> according to the PCI ordering rules.

This is not how I see that thread concluding..

The device reclaim problem belongs solely to VFIO, not KVM. VFIO must
ensure global ordering of access before the VMA is unmaped and access
after, that includes ordering whatever mechanism the VFIO driver uses
for reset.

If there are quirky SOCs, or non-PCI devices that need something
stronger than the TLBI/etc sequence it should be fixed in VFIO (or
maybe even the arch code), not by blocking NORMAL_NC in the KVM. Such
a quirky SOC would broadly have security issues beyond KVM.

> It is worth noting that currently, to map devices MMIO space to user
> space in a device pass-through use case the VFIO framework applies memory
> attributes derived from pgprot_noncached() settings applied to VMAs, which

Sometimes. VFIO uses a mix of pgprot_noncached and pgprot_device. AFAIK
we should change to to always use pgprot_device..

Thanks,
Jason