Re: [5.17 regression] "x86/PCI: Ignore E820 reservations for bridge windows on newer systems" breaks suspend/resume

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 2/9/22 17:01, Mika Westerberg wrote:
> Hi Hans,
> 
> Thanks for looking into this!
> 
> On Wed, Feb 09, 2022 at 04:12:32PM +0100, Hans de Goede wrote:
>> An alternative, much more elaborate fix would be to:
>>
>> 1. Add E820_TYPE_MMIO and E820_TYPE_MMIO_PCI_BRIDGE_WINDOW types to
>>    enum e820_type and modify the 2 places which check for type == reserved
>>    to treat these both as reserved too, so as to not have any functional
>>    changes there
>>
>> 2. Modify the code building e820 tables from the EFI memmap to use
>>    E820_TYPE_MMIO for MMIO EFI memmap entries and make e820_nomerge()
>>    treat E820_TYPE_MMIO as non mergable to retain these as is
>>
>> 3. Modify pci_acpi_root_prepare_resources() to compare ACPI provided
>>    bridge mmio windows against E820_TYPE_MMIO e820_Table entries and
>>    if there is an exact match (as is the case on the Yoga C940)
>>    then change the e820 type to E820_TYPE_MMIO_PCI_BRIDGE_WINDOW
>>
>>    This means that we are now very narrowly treating EFI MMIO marked
>>    regions special, in the special case where they are a 1:1 map
>>    to an ACPI PCI bridge window resource.
>>
>>    Note since we treat both E820_TYPE_MMIO and E820_TYPE_MMIO_PCI_BRIDGE_WINDOW
>>    identical to the old RESERVED everywhere there is no functional change
>>    here.
>>
>> 4. Modify arch/x86/kernel/resource.c: remove_e820_regions() to skip
>>    e820 table entries with a type of E820_TYPE_MMIO_PCI_BRIDGE_WINDOW,
>>    this would actually be a functional change and should fix the
>>    issues we are trying to fix.
>>
>> Note an alternative would be to skip having the extra E820_TYPE_MMIO_PCI_BRIDGE_WINDOW
>> type and to skip step 3. above. That would boil down to doing the same
>> as your original patch in a somewhat cleaner manner.
> 
> I agree and my vote goes for this last alternative (e.g skipping step 3).
> That would also make it slightly easier to follow the logic if someone
> in the future needs to investigate possible issues around this, I think.

As mentioned in my email from 10 seconds ago I think a better simpler
fix would be to just do:

diff --git a/arch/x86/kernel/resource.c b/arch/x86/kernel/resource.c
index 9b9fb7882c20..18656f823764 100644
--- a/arch/x86/kernel/resource.c
+++ b/arch/x86/kernel/resource.c
@@ -28,6 +28,10 @@ static void remove_e820_regions(struct resource *avail)
 	int i;
 	struct e820_entry *entry;
 
+	/* Only remove E820 reservations on classic BIOS boot */
+	if (efi_enabled(EFI_MEMMAP))
+		return;
+
 	for (i = 0; i < e820_table->nr_entries; i++) {
 		entry = &e820_table->entries[i];
 

I'm curious what you think of that?

Regards,

Hans