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Message-ID: <20240228194801.2299-1-ankita@nvidia.com>
Date: Wed, 28 Feb 2024 19:48:01 +0000
From: <ankita@...dia.com>
To: <ankita@...dia.com>, <jgg@...dia.com>, <alex.williamson@...hat.com>,
<yishaih@...dia.com>, <shameerali.kolothum.thodi@...wei.com>,
<kevin.tian@...el.com>
CC: <aniketa@...dia.com>, <cjia@...dia.com>, <kwankhede@...dia.com>,
<targupta@...dia.com>, <vsethi@...dia.com>, <acurrid@...dia.com>,
<apopple@...dia.com>, <jhubbard@...dia.com>, <danw@...dia.com>,
<rrameshbabu@...dia.com>, <zhiw@...dia.com>, <anuaggarwal@...dia.com>,
<mochs@...dia.com>, <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: [PATCH v1 1/1] vfio/nvgrace-gpu: Convey kvm that the device is wc safe
From: Ankit Agrawal <ankita@...dia.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_WB | NORMAL_NC | NORMAL_NC
NORMAL_WT | NORMAL_NC | NORMAL_NC
NORMAL_NC | NORMAL_NC | NORMAL_NC
NORMAL_WB | DEVICE_nGnRE | DEVICE_nGnRE
NORMAL_WT | DEVICE_nGnRE | DEVICE_nGnRE
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. Setting S2 as MT_S2_FWB_NORMAL_NC
provides the desired behavior (uncached, unaligned access) for resmem.
Such setting is extended to the usemem as a middle-of-the-road
setting to take it closer to the desired final system memory
characteristics (cached, unaligned). This will eventually be
fixed with the ongoing proposal [4].
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/20230907181459.18145-2-ankita@nvidia.com/ [4]
Cc: Alex Williamson <alex.williamson@...hat.com>
Cc: Kevin Tian <kevin.tian@...el.com>
Cc: Jason Gunthorpe <jgg@...dia.com>
Cc: Vikram Sethi <vsethi@...dia.com>
Cc: Zhi Wang <zhiw@...dia.com>
Signed-off-by: Ankit Agrawal <ankita@...dia.com>
---
drivers/vfio/pci/nvgrace-gpu/main.c | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)
diff --git a/drivers/vfio/pci/nvgrace-gpu/main.c b/drivers/vfio/pci/nvgrace-gpu/main.c
index 25814006352d..5539c9057212 100644
--- a/drivers/vfio/pci/nvgrace-gpu/main.c
+++ b/drivers/vfio/pci/nvgrace-gpu/main.c
@@ -181,6 +181,24 @@ static int nvgrace_gpu_mmap(struct vfio_device *core_vdev,
vma->vm_pgoff = start_pfn;
+ /*
+ * The VM_ALLOW_ANY_UNCACHED VMA flag is implemented for ARM64,
+ * allowing KVM stage 2 device mapping attributes to use Normal-NC
+ * rather than DEVICE_nGnRE, which allows guest mappings
+ * supporting write-combining attributes (WC). This also
+ * unlocks memory-like operations such as unaligned accesses.
+ * This setting suits the fake BARs as they are expected to
+ * demonstrate such properties within the guest.
+ *
+ * ARM does not architecturally guarantee this is safe, and indeed
+ * some MMIO regions like the GICv2 VCPU interface can trigger
+ * uncontained faults if Normal-NC is used. The nvgrace-gpu
+ * however is safe in that the platform guarantees that no
+ * action taken on the MMIO mapping can trigger an uncontained
+ * failure. Hence VM_ALLOW_ANY_UNCACHED is set in the VMA flags.
+ */
+ vm_flags_set(vma, VM_ALLOW_ANY_UNCACHED);
+
return 0;
}
--
2.34.1
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