Smart Access Memory vs Qemu/KVM

GlemSom · March 4, 2021, 5:42pm

Does anyone know if Smart Access Memory can be used from a guest running inside qemu/kvm ?
I would assume this is a CPU flag, which might or might not be visible to the guest… How-ever, I have no clue if BAR resize from within the guest will ever work ?

reavessm · March 5, 2021, 1:07pm

I would imagine if you did if you passed the GPU through to the guest, and set the cpu topology to host-passthrough, then it SHOULD be fine, assuming of course the feature is enabled in the BIOS, but I would still like to see if anybody actually gets it to work. Also, FWIW, there are some cpuflags that still have to be manually enabled, even after using host-passthrough, so it might take some tinkering

lOneT · March 15, 2021, 3:23am

I would like to know if this is working. I think this is an interesting question.

anon27075190 · March 15, 2021, 9:34pm

@wendell You have some insight here or hardware you could use for a quick test. Unfortunately I do not have a GPU that new.

GlemSom · March 18, 2021, 5:33am

Digging through the QEMU source-code, it would seem BAR Resize is being hidden from the guest (and not supported):

github.com

qemu/qemu/blob/2255564fd21059960966b47212def9069cb56077/hw/vfio/pci.c#L2089


    size = vfio_ext_cap_max_size(config, next);
    /* Use emulated next pointer to allow dropping extended caps */
    pci_long_test_and_set_mask(vdev->emulated_config_bits + next,
                               PCI_EXT_CAP_NEXT_MASK);
    switch (cap_id) {
    case 0: /* kernel masked capability */
    case PCI_EXT_CAP_ID_SRIOV: /* Read-only VF BARs confuse OVMF */
    case PCI_EXT_CAP_ID_ARI: /* XXX Needs next function virtualization */
    case PCI_EXT_CAP_ID_REBAR: /* Can't expose read-only */
        trace_vfio_add_ext_cap_dropped(vdev->vbasedev.name, cap_id, next);
        break;
    default:
        pcie_add_capability(pdev, cap_id, cap_ver, next, size);
    }
}
/* Cleanup chain head ID if necessary */
if (pci_get_word(pdev->config + PCI_CONFIG_SPACE_SIZE) == 0xFFFF) {

I would suspect this means Smart Access Memory will not work for a VFIO setup then.
I don’t have the required hardware to confirm this tough.

Pixo · March 18, 2021, 7:03am

Tested this on Qemu 5.2.0 with 6900XT, 5950X and MSI X570 CREATION with Agesa 1.2.0.0.
The GPU is working and there is a video output but the driver will not work when BAR Resize is enabled.
So its end up with using the generic GPU driver.

lOneT · March 21, 2021, 5:46am

Thanks Pixo for your input. I will have to revisit this and will keep an eye out for any updates. Good job with the source dive GlemSom. This may interest someone enough to dig further.

Pixo · March 23, 2021, 3:27pm

So patching the line disabling REBAR capabilities out will allow the driver to work if SAM is enabled.
And according to GPU-Z it is.
But the system is unstable so something is not working correctly.