RE: [PATCH] PCI: hv: Allocate MMIO from above 4GB for the config window

[Dexuan Cui]

> From: Michael Kelley <mhklinux@xxxxxxxxxxx>
> Sent: Friday, January 23, 2026 10:28 AM
>  ...
> One more thought here: Is commit 96959283a58d relevant? The
> commit message describes a scenario where vmbus_reserve_fb()
> doesn't do anything because CONFIG_SYSFB is not set. Looking at
> the code for vmbus_reserve_fb(), it doing nothing might imply that
> screen_info.lfb_base is 0. But when CONFIG_SYSFB is not set,
> screen_info.lfb_base is just ignored, with the same result. This behavior
> started with the 6.7 kernel due to commit a07b50d80ab6.
> 
> Note that commit 96959283a58d has a follow-on to correct a
> problem when CONFIG_EFI is not set.  See commit 7b89a44b2e8c.
> If there's a reason to backport 96959283a58d, also get
> 7b89a44b2e8c.
> 
> Michael

In my opinion,
96959283a58d ("Drivers: hv: Always select CONFIG_SYSFB for Hyper-V guests")
is not a good fix for a07b50d80ab6: the commit message of a07b50d80ab6
says "the vmbus_drv code marks the original EFI framebuffer as reserved, but
this is not required if there is no sysfb" -- IMO the message is incorrect.

Even if CONFIG_SYSFB is not set, we still need to reserve the framebuffer
MMIO range, because we need to make sure that hv_pci doesn't allocate
MMIO from there.

96959283a58d adds "select SYSFB if !HYPERV_VTL_MODE", but we can
still manually unset CONFIG_SYSFB (I happened to do this when debugging
the kdump issue), and hv_pci won't work.

IMO vmbus_reserve_fb() should unconditionally reserve the frame buffer
MMIO range. I'll post a patch like this:

--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -2395,10 +2398,8 @@ static void __maybe_unused vmbus_reserve_fb(void)

        if (efi_enabled(EFI_BOOT)) {
                /* Gen2 VM: get FB base from EFI framebuffer */
-               if (IS_ENABLED(CONFIG_SYSFB)) {
-                       start = sysfb_primary_display.screen.lfb_base;
-                       size = max_t(__u32, sysfb_primary_display.screen.lfb_size, 0x800000);
-               }
+               start = sysfb_primary_display.screen.lfb_base;
+               size = max_t(__u32, sysfb_primary_display.screen.lfb_size, 0x800000);
        } else {
                /* Gen1 VM: get FB base from PCI */
                pdev = pci_get_device(PCI_VENDOR_ID_MICROSOFT,


diff --git a/drivers/hv/Kconfig b/drivers/hv/Kconfig
index 7937ac0cbd0f..78d7f8c66278 100644
--- a/drivers/hv/Kconfig
+++ b/drivers/hv/Kconfig
@@ -9,7 +9,6 @@ config HYPERV
        select PARAVIRT
        select X86_HV_CALLBACK_VECTOR if X86
        select OF_EARLY_FLATTREE if OF
-       select SYSFB if EFI && !HYPERV_VTL_MODE
        select IRQ_MSI_LIB if X86
        help
          Select this option to run Linux as a Hyper-V client operating

Thanks,
Dexuan