Re: [PATCH v14 29/44] arm64: RMI: Runtime faulting of memory

[Gavin Shan <gshan@redhat.com>]

On 6/27/26 2:44 AM, Lorenzo Pieralisi wrote:
> On Fri, Jun 26, 2026 at 09:43:03PM +1000, Gavin Shan wrote:
>> On 6/26/26 6:47 PM, Suzuki K Poulose wrote:
>>> On 26/06/2026 08:43, Gavin Shan wrote:
>>>> On 6/26/26 1:58 AM, Suzuki K Poulose wrote:
>>>>> On 25/06/2026 14:53, Gavin Shan wrote:
>>>>>> On 6/6/26 12:35 AM, Lorenzo Pieralisi wrote:
>>>>>>> On Fri, Jun 05, 2026 at 06:11:11PM +1000, Gavin Shan wrote:
>>>>>>>> On 6/5/26 5:28 PM, Lorenzo Pieralisi wrote:
>>>>>>>>> On Fri, Jun 05, 2026 at 04:23:15PM +1000, Gavin Shan wrote:
>>>>
>>>> [...]
>>>>
>>>>>>>>
>>>>>>>> I tried to rebase Jean's latest QEMU series [1] to upstream QEMU, and found
>>>>>>>> that memory slots backed by THP are broken. With THP disabled on the host and
>>>>>>>> other fixes (mentioned in my prevous replies) applied on the top of this (v14)
>>>>>>>> series, I'm able to boot a realm guest with rebased QEMU series [2], plus more
>>>>>>>> fxies on the top.
>>>>>>>>
>>>>>>>> [1] https://git.codelinaro.org/linaro/dcap/qemu.git  (branch: cca/ latest)
>>>>>>>> [2] https://git.qemu.org/git/qemu.git                (branch: cca/ gavin)
>>>>>>>>
>>>>>>>> Lorenzo, You may be saying there is someone making QEMU to support ARM/CCA?
>>>>>>>
>>>>>>> Mathieu and I are working on that yes and with Steven/Suzuki to fix the THP
>>>>>>> issues you pointed out above.
>>>>>>>
>>>>>>>> If so, I'm not sure if there is a QEMU repository for me to try?
>>>>>>>
>>>>>>> We should be able to submit patches by end of June - we shall let you know
>>>>>>> whether we can make something available earlier.
>>>>>>>
>>>>>>
>>>>>> Not sure if there are other known issues in this series. It seems the stage2
>>>>>> page fault handling on the shared space isn't working well. In my test, the
>>>>>> vring (struct vring_desc) of virtio-net-pci is updated by the guest, and the
>>>>>> data isn't seen by QEMU, I'm suspecting if the host-page-frame-number is properly
>>>>>> resolved in the s2 page fault handler for shared (unprotected) space.
>>>>>>
>>>>>> - I rebased Jean's latest qemu branch to the upstream qemu;
>>>>>>
>>>>>> - On the host, which is emulated by qemu/tcg, the THP (transparent huge page) is
>>>>>>     disabled.
>>>>>>
>>>>>> - On the guest, I can see the virtio vring (struct vring_desc) is updated. The
>>>>>>     S1 page-table entry looks correct because the corresponding physical address
>>>>>>     0x10046880000 is a sane shared (unprotected) space address.
>>>>>>
>>>>>>     [   52.094143] software IO TLB: Memory encryption is active and system is using DMA bounce buffers
>>>>>>     [   52.289746] virtqueue_add_desc_split: desc[0]@0xffff000006880000, [00000100b983f000  00000640  0002  0001]
>>>>>>     [   52.432150] PTE 0x00e8010046880707 at address 0xffff000006880000
>>>>>>
>>>>>> - On the host, the s2 page-table-entry is unmapped due to attribute transition (private -> shared).
>>>>>>     A subsequent S2 page fault is raised against the adress and the s2 page-table-entry is built.
>>>>>>
>>>>>>     [  109.259077] ====> realm_unmap_shared_range: tracked_unprot_addr=0x10046880000
>>>>>>     [  109.260249] realm_unmap_shared_range: unmapped shared range at 0x10046880000
>>>>>>     [  109.317786] realm_unmap_shared_range: unmapped shared range at 0x10046880000
>>>>>>     [  109.629939] ====> kvm_handle_guest_abort: fault_ipa=0x10046880000, esr=0x92000007
>>>>>>     [  109.630245] realm_map_non_secure: ipa=0x10046880000, pfn=0xb8b59, size=0x1000, prot=0xf
>>>>>>     [  109.630331] realm_map_non_secure: ipa=0x10046880000, ipa_top=0x10046881000, flags=0x1e0001, range_desc=0xb8b59004
>>>>>
>>>>> Are you able to correlate the order of the transitions and the Guest
>>>>> access with RMM log ? We haven't seen this from our end. We are aware
>>>>> of permission fault issues with Unprotected IPA when backing the memslot
>>>>> with MAP_PRIVATE areas. But this looks different.
>>>>>
>>>>> Lorenzo, have you run into this ?
>>>>>
>>>>
>>>> It's hard to correlate the order since the logs are collected from two separate
>>>> consoles. For the write permission, I add code to the host where the permission
>>>> is always added for all s2 page faults in the shared space. Otherwise, qemu can
>>>> be killed by -EFAULT or similar error.
>>>
>>> This is the problem. We can't add WRITE permission by default. I believe
>>> you may have MAP_PRIVATE mapping and it has to be mapped as READ only
>>> and on a permission fault, we replace it with a writable page. By
>>> overriding the WRITE permission, you let the guest write to a page
>>> that may not be seen by the VMM.
>>>
>>> We identified this as a bug in the KVM driver in this series (reported
>>> by Lorenzo) and there is a corresponding tf-RMM change that is required
>>> to get this working. So, please could you wait until the next series
>>> when this will be addressed ? Or you could switch to using MAP_SHARED
>>> for the "shared" memory in the memslot.
>>>
>>
>> Exactly. the syntax for MAP_PRIVATE is broken if the write permission is
>> enforced for a read fault in the shared space. In my case, the host page can
>> be the zero page and eventually multiple s2 page-table entries (for multiple
>> unprotected or shared pages) point to the zero page. It's why clearing the
>> 3rd queue (Ctrl queue) also clears the first queue (Rx queue) in my case.
>>
>> Yes, this issue can be avoid by using a shared memory backend in qemu, something
>> like below. With this, I'm able to see virtio-net-pci starts to work...
>>
>>      -object memory-backend-ram,id=mem0,size=2G,share=yes
> 
> Yes, as Suzuki said that's what we have been fixing. QEmu patches
> will be on the mailing lists very shortly - the KVM/tf-RMM fixes
> to make MAP_PRIVATE work will be included in the next posting.
> 
> Feel free to drop your QEmu command line so that I can give it
> a shot and check whether the fixes solve the problem you hit
> (I think so because that's precisely the kind of issue I got
> into when I started debugging THP/MAP_PRIVATE but it is better
> to check).
> 

The virtio-net-pci doesn't work with the following command lines. The guest
kernel image is built from upstream kernel (v7.1.rc7).

     qemu-system-aarch64 -enable-kvm -object rme-guest,id=rme0,             \
     -machine virt,gic-version=3,confidential-guest-support=rme0            \
     -cpu host,pmu=off                                                      \
     -smp maxcpus=2,cpus=2,sockets=1,clusters=1,cores=1,threads=2           \
     -m 2G -object memory-backend-ram,id=mem0,size=2G                       \
     -numa node,nodeid=0,cpus=0-1,memdev=mem0                               \
     -serial mon:stdio -monitor none -nographic -nodefaults                 \
     -kernel /mnt/linux/arch/arm64/boot/Image                               \
     -initrd /mnt/buildroot/output/images/rootfs.cpio.xz                    \
     -append earlycon=pl011,mmio,0x10009000000                              \
     -device pcie-root-port,bus=pcie.0,chassis=1,id=pcie.1                  \
     -device pcie-root-port,bus=pcie.0,chassis=2,id=pcie.2                  \
     -device pcie-root-port,bus=pcie.0,chassis=3,id=pcie.3                  \
     -device pcie-root-port,bus=pcie.0,chassis=4,id=pcie.4                  \
     -netdev tap,id=tap1,vhost=on,script=/etc/qemu-ifup,downscript=/etc/qemu-ifdown  \
     -device virtio-net-pci,bus=pcie.2,netdev=tap1,mac=b8:3f:d2:1d:3e:c0

The virtio-net-pci starts to work with the shareable memory-backend.

     -object memory-backend-ram,id=mem0,size=2G,share=yes

Note that THP is disabled on my host.

     root@host:~# cat /sys/kernel/mm/transparent_hugepage/enabled
     always madvise [never]

Thanks,
Gavin

> Thanks,
> Lorenzo
>