From: Alex Williamson <alex.williamson@redhat.com>
To: <ankita@nvidia.com>, Oliver Upton <oliver.upton@linux.dev>
Cc: <jgg@nvidia.com>, <yishaih@nvidia.com>,
<shameerali.kolothum.thodi@huawei.com>, <kevin.tian@intel.com>,
<aniketa@nvidia.com>, <cjia@nvidia.com>, <kwankhede@nvidia.com>,
<targupta@nvidia.com>, <vsethi@nvidia.com>, <acurrid@nvidia.com>,
<apopple@nvidia.com>, <jhubbard@nvidia.com>, <danw@nvidia.com>,
<rrameshbabu@nvidia.com>, <zhiw@nvidia.com>,
<anuaggarwal@nvidia.com>, <mochs@nvidia.com>,
<kvm@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
kvmarm@lists.linux.dev
Subject: Re: [PATCH v2 1/1] vfio/nvgrace-gpu: Convey kvm to map device memory region as noncached
Date: Tue, 5 Mar 2024 16:12:56 -0700 [thread overview]
Message-ID: <20240305161256.09bda6c4.alex.williamson@redhat.com> (raw)
In-Reply-To: <20240229193934.2417-1-ankita@nvidia.com>
On Thu, 29 Feb 2024 19:39:34 +0000
<ankita@nvidia.com> wrote:
> From: Ankit Agrawal <ankita@nvidia.com>
>
> The NVIDIA Grace Hopper GPUs have device memory that is supposed to be
> used as a regular RAM. It is accessible through CPU-GPU chip-to-chip
> cache coherent interconnect and is present in the system physical
> address space. The device memory is split into two regions - termed
> as usemem and resmem - in the system physical address space,
> with each region mapped and exposed to the VM as a separate fake
> device BAR [1].
>
> Owing to a hardware defect for Multi-Instance GPU (MIG) feature [2],
> there is a requirement - as a workaround - for the resmem BAR to
> display uncached memory characteristics. Based on [3], on system with
> FWB enabled such as Grace Hopper, the requisite properties
> (uncached, unaligned access) can be achieved through a VM mapping (S1)
> of NORMAL_NC and host mapping (S2) of MT_S2_FWB_NORMAL_NC.
>
> KVM currently maps the MMIO region in S2 as MT_S2_FWB_DEVICE_nGnRE by
> default. The fake device BARs thus displays DEVICE_nGnRE behavior in the
> VM.
>
> The following table summarizes the behavior for the various S1 and S2
> mapping combinations for systems with FWB enabled [3].
> S1 | S2 | Result
> NORMAL_NC | NORMAL_NC | NORMAL_NC
> NORMAL_NC | DEVICE_nGnRE | DEVICE_nGnRE
>
> Recently a change was added that modifies this default behavior and
> make KVM map MMIO as MT_S2_FWB_NORMAL_NC when a VMA flag
> VM_ALLOW_ANY_UNCACHED is set [4]. Setting S2 as MT_S2_FWB_NORMAL_NC
> provides the desired behavior (uncached, unaligned access) for resmem.
>
> To use VM_ALLOW_ANY_UNCACHED flag, the platform must guarantee that
> no action taken on the MMIO mapping can trigger an uncontained
> failure. The Grace Hopper satisfies this requirement. So set
> the VM_ALLOW_ANY_UNCACHED flag in the VMA.
>
> Applied over next-20240227.
> base-commit: 22ba90670a51
>
> Link: https://lore.kernel.org/all/20240220115055.23546-4-ankita@nvidia.com/ [1]
> Link: https://www.nvidia.com/en-in/technologies/multi-instance-gpu/ [2]
> Link: https://developer.arm.com/documentation/ddi0487/latest/ section D8.5.5 [3]
> Link: https://lore.kernel.org/all/20240224150546.368-1-ankita@nvidia.com/ [4]
>
> Cc: Alex Williamson <alex.williamson@redhat.com>
> Cc: Kevin Tian <kevin.tian@intel.com>
> Cc: Jason Gunthorpe <jgg@nvidia.com>
> Cc: Vikram Sethi <vsethi@nvidia.com>
> Cc: Zhi Wang <zhiw@nvidia.com>
> Signed-off-by: Ankit Agrawal <ankita@nvidia.com>
> ---
> drivers/vfio/pci/nvgrace-gpu/main.c | 11 ++++++++++-
> 1 file changed, 10 insertions(+), 1 deletion(-)
Applied to vfio next branch for v6.9.
Oliver, FYI I did merge the branch you provided in [1] for this, thanks
for the foresight in providing that.
Alex
[1]https://lore.kernel.org/all/170899100569.1405597.5047894183843333522.b4-ty@linux.dev/
>
> diff --git a/drivers/vfio/pci/nvgrace-gpu/main.c b/drivers/vfio/pci/nvgrace-gpu/main.c
> index 25814006352d..a7fd018aa548 100644
> --- a/drivers/vfio/pci/nvgrace-gpu/main.c
> +++ b/drivers/vfio/pci/nvgrace-gpu/main.c
> @@ -160,8 +160,17 @@ static int nvgrace_gpu_mmap(struct vfio_device *core_vdev,
> * The carved out region of the device memory needs the NORMAL_NC
> * property. Communicate as such to the hypervisor.
> */
> - if (index == RESMEM_REGION_INDEX)
> + if (index == RESMEM_REGION_INDEX) {
> + /*
> + * The nvgrace-gpu module has no issues with uncontained
> + * failures on NORMAL_NC accesses. VM_ALLOW_ANY_UNCACHED is
> + * set to communicate to the KVM to S2 map as NORMAL_NC.
> + * This opens up guest usage of NORMAL_NC for this mapping.
> + */
> + vm_flags_set(vma, VM_ALLOW_ANY_UNCACHED);
> +
> vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
> + }
>
> /*
> * Perform a PFN map to the memory and back the device BAR by the
next parent reply other threads:[~2024-03-05 23:13 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20240229193934.2417-1-ankita@nvidia.com>
2024-03-05 23:12 ` Alex Williamson [this message]
2024-03-06 1:46 ` [PATCH v2 1/1] vfio/nvgrace-gpu: Convey kvm to map device memory region as noncached Ankit Agrawal
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