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From: Krister Johansen To: Dexuan Cui Cc: kys@microsoft.com, haiyangz@microsoft.com, wei.liu@kernel.org, longli@microsoft.com, linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org, mhklinux@outlook.com, matthew.ruffell@canonical.com, hargar@linux.microsoft.com, stable@vger.kernel.org Subject: Re: [PATCH v2] Drivers: hv: vmbus: Improve the logic of reserving fb_mmio on Gen2 VMs Message-ID: References: <20260505004846.193441-1-decui@microsoft.com> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260505004846.193441-1-decui@microsoft.com> X-Migadu-Flow: FLOW_OUT On Mon, May 04, 2026 at 05:48:46PM -0700, Dexuan Cui wrote: > If vmbus_reserve_fb() in the kdump/kexec kernel fails to properly reserve > the framebuffer MMIO range (which is below 4GB) due to a Gen2 VM's > screen.lfb_base being zero [1], there is an MMIO conflict between the > drivers hyperv-drm and pci-hyperv: when the driver pci-hyperv's > hv_pci_allocate_bridge_windows() calls vmbus_allocate_mmio() to get a > 32-bit MMIO range, it may get an MMIO range that overlaps with the > framebuffer MMIO range, and later hv_pci_enter_d0() fails with an > error message "PCI Pass-through VSP failed D0 Entry with status" since > the host thinks that PCI devices must not use MMIO space that the > host has assigned to the framebuffer. > > This is especially an issue if pci-hyperv is built-in and hyperv-drm is > built as a module. Consequently, the kdump/kexec kernel fails to detect > PCI devices via pci-hyperv, and may fail to mount the root file system, > which may reside in a NVMe disk. The issue described here has existed > for SR-IOV VF NICs since day one of the pci-hyperv driver, and has been > worked around on x64 when possible. With the recent introduction of > ARM64 VMs that boot from NVMe, there is no workaround, so we need a > formal fix. > > On Gen2 VMs, if the screen.lfb_base is 0 in the kdump/kexec kernel [1], > fall back to the low MMIO base, which should be equal to the framebuffer > MMIO base [2] (the statement is true according to my testing on x64 > Windows Server 2016, and on x64 and ARM64 Windows Server 2025 and on > Azure. I checked with the Hyper-V team and they said the statement should > continue to be true for Gen2 VMs). In the first kernel, screen.lfb_base > is not 0; if the user specifies a very high resolution, it's not enough > to only reserve 8MB: in this case, reserve half of the space below 4GB, > but cap the reservation to 128MB, which is the required framebuffer size > of the highest resolution 7680*4320 supported by Hyper-V. > > While at it, fix the comparison "end > VTPM_BASE_ADDRESS" by changing > the > to >=. Here the 'end' is an inclusive end (typically, it's > 0xFFFF_FFFF for the low MMIO range). > > Note: vmbus_reserve_fb() now also reserves an MMIO range at the beginning > of the low MMIO range on CVMs, which have no framebuffers (the > 'screen.lfb_base' in vmbus_reserve_fb() is 0 for CVMs), just in case the > host might treat the beginning of the low MMIO range specially [4]. BTW, > the OpenHCL kernel is not affected by the change, because that kernel > boots with DeviceTree rather than ACPI (so vmbus_reserve_fb() won't run > there), and there is no framebuffer device for that kernel. > > Note: normally Gen1 VMs don't have the MMIO conflict issue because the > framebuffer MMIO range (which is hardcoded to base=4GB-128MB and > size=64MB for Gen1 VMs by the host) is always reported via the legacy PCI > graphics device's BAR, so the kdump/kexec kernel can reserve the 64MB > MMIO range; however, if the VM is configured to use a very high resolution > and the required framebuffer size exceeds 64MB (AFAIK, in practice, this > isn't a typical configuration by users), the hyperv-drm driver may need to > allocate an MMIO range above 4GB and change the framebuffer MMIO location > to the allocated MMIO range -- in this case, there can still be issues [3] > which can't be easily fixed: any possible affected Gen1 users would have > to use a resolution whose framebuffer size is <= 64MB, or switch to Gen2 > VMs. > > [1] https://lore.kernel.org/all/SA1PR21MB692176C1BC53BFC9EAE5CF8EBF51A@SA1PR21MB6921.namprd21.prod.outlook.com/ > [2] https://lore.kernel.org/all/SA1PR21MB69218F955B62DFF62E3E88D2BF222@SA1PR21MB6921.namprd21.prod.outlook.com/ > [3] https://lore.kernel.org/all/SA1PR21MB69213486F821CA5A2C793C81BF342@SA1PR21MB6921.namprd21.prod.outlook.com/ > [4] https://lore.kernel.org/all/SN6PR02MB415726B17D5A6027CD1717E8D4342@SN6PR02MB4157.namprd02.prod.outlook.com/ > > Fixes: 4daace0d8ce8 ("PCI: hv: Add paravirtual PCI front-end for Microsoft Hyper-V VMs") > CC: stable@vger.kernel.org > Signed-off-by: Dexuan Cui > --- Thanks for the updated patch. I re-tested this on a D2pdsv6 and was able to confirm that without the patch the NIC drivers in the dump environment didn't attach because of a PCI conflict. With the patch the drivers attached and it was possible to successfully collect a kdump. Tested-by: Krister Johansen -K