Re: [PATCH v7 0/4] kvm: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory

[David Hildenbrand]

On 11.02.24 18:47, ankita@xxxxxxxxxx wrote:
> From: Ankit Agrawal <ankita@xxxxxxxxxx>

Hi,

> Currently, KVM for ARM64 maps at stage 2 memory that is considered device
> with DEVICE_nGnRE memory attributes; this setting overrides (per
> ARM architecture [1]) any device MMIO mapping present at stage 1,
> resulting in a set-up whereby a guest operating system cannot
> determine device MMIO mapping memory attributes on its own but
> it is always overridden by the KVM stage 2 default.
>
> This set-up does not allow guest operating systems to select device
> memory attributes independently from KVM stage-2 mappings
> (refer to [1], "Combining stage 1 and stage 2 memory type attributes"),
> which turns out to be an issue in that guest operating systems
> (e.g. Linux) may request to map devices MMIO regions with memory
> attributes that guarantee better performance (e.g. gathering
> attribute - that for some devices can generate larger PCIe memory
> writes TLPs) and specific operations (e.g. unaligned transactions)
> such as the NormalNC memory type.
>
> The default device stage 2 mapping was chosen in KVM for ARM64 since
> it was considered safer (i.e. it would not allow guests to trigger
> uncontained failures ultimately crashing the machine) but this
> turned out to be asynchronous (SError) defeating the purpose.
>
> For these reasons, relax the KVM stage 2 device memory attributes
> from DEVICE_nGnRE to Normal-NC.
>
> Generalizing to other devices may be problematic, however. E.g.
> GICv2 VCPU interface, which is effectively a shared peripheral, can
> allow a guest to affect another guest's interrupt distribution. Hence
> limit the change to VFIO PCI as caution. This is achieved by
> making the VFIO PCI core module set a flag that is tested by KVM
> to activate the code. This could be extended to other devices in
> the future once that is deemed safe.

I still have to digest some of the stuff I learned about this issue, 
please bear with me :)

(1) PCI BARs might contain mixtures of RAM and MMIO, the exact 
locations/semantics within a BAR are only really known to the actual 
device driver.

We must not unconditionally map PFNs "the wrong way", because it can 
have undesired side effects. Side effects might include 
read-speculation, that can be very problematic with MMIO regions.

The safe way (for the host) is DEVICE_nGnRE. But that is actually 
problematic for performance (where we want WC?) and unaligned accesses 
(where we want NC?).

We can trigger both cases right now inside VMs, where we want the device 
driver to actually make the decision.


(2) For a VM, that device driver lives inside the VM, for DPDK and 
friends, it lives in user space. They have this information.

We only focus here on optimizing (fixing?) the mapping for VMs, DPDK is 
out of the picture. So we want to allow the VM to achieve a WC/NC 
mapping by using a relaxed (NC) mapping in stage-1. Whatever is set in 
stage-2 wins.


(3) vfio knows whether using WC (and NC?) could be problematic, and must 
forbid it, if that is the case. There are cases where we could otherwise 
cause harm (bring down the host?). We must keep mapping the memory as 
DEVICE_nGnRE when in doubt.


Now, what the new mmap() flag does is tell the world "using the wrong 
mapping type cannot bring down the host", and KVM uses that to use a 
different mapping type (NC) in stage-1 as setup by vfio in the user 
space page tables.

I was trying to find ways of avoiding a mmap() flag and was hoping that 
we could just use a PTE bit that does not have semantics in VM_PFNMAP 
mappings. Unfortunately, arm64 does not support uffd-wp, which I had in 
mind, so it's not that easy.


Further, I was wondering if there would be a way to let DPDK similarly 
benefit, because it looks like we are happily ignoring that (I was told 
they apply some hacks to work around that).


In essence, user space knows how it will consume that memory: QEMU wants 
to mmap() it only to get it into stage-1 and not access it via the user 
page tables. DPDK wants to mmap() it to actually access it from user space.


So I am curious, is the following problematic, and why:

(a) User space tells VFIO which parts of a BAR it would like to have 
mapped differently. For QEMU, this would mean, requesting a NC mapping 
for the whole BAR. For DPDK, it could mean requesting different types 
for parts of a BAR.

(b) VFIO decides if it is safe to use a relaxed mapping. If in doubt, it 
falls back to existing (legacy) handling -- DEVICE_nGnRE.

(c) KVM simply uses the existing mapping type instead of diverging from 
the one in the user space mapping.


That would mean, that we would map NC already in QEMU. I wonder if that 
could be a problem with read speculation, even if QEMU never really 
accesses that mmap'ed region.

Something like that would of course require user space changes. Handling 
it without such changes (ignoring DPDK of course) would require some 
information exchange between KVM and vfio, like the mmap flag proposed.

--
Cheers,

David / dhildenb