Re: [PATCH] hvmloader/PCI: skip huge BARs in certain calculations

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.03.2024 10:31, Roger Pau Monné wrote:
> On Mon, Mar 04, 2024 at 08:32:22AM +0100, Jan Beulich wrote:
>> BARs of size 2Gb and up can't possibly fit below 4Gb: Both the bottom of
>> the lower 2Gb range and the top of the higher 2Gb range have special
>> purpose. Don't even have them influence whether to (perhaps) relocate
>> low RAM.
>>
>> Reported-by: Neowutran <xen@xxxxxxxxxxxxx>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks.

>> --- a/tools/firmware/hvmloader/pci.c
>> +++ b/tools/firmware/hvmloader/pci.c
>> @@ -33,6 +33,13 @@ uint32_t pci_mem_start = HVM_BELOW_4G_MM
>>  const uint32_t pci_mem_end = RESERVED_MEMBASE;
>>  uint64_t pci_hi_mem_start = 0, pci_hi_mem_end = 0;
>>  
>> +/*
>> + * BARs larger than this value are put in 64-bit space unconditionally.  
>> That
>> + * is, such BARs also don't play into the determination of how big the 
>> lowmem
>> + * MMIO hole needs to be.
>> + */
>> +#define HUGE_BAR_THRESH GB(1)
> 
> I would maybe name this `BAR_RELOCATE_THRESH, `HUGE_BAR` is too
> generic IMO.

BAR_RELOC_THRESH it is then.

> And also use 256Mb instead of 1GB, but just having a limit is good
> enough, we can further tune it afterwards.

As indicated in an earlier reply, I now firmly think that if we want to
do so, it wants doing separately / incrementally.

>> @@ -446,8 +455,9 @@ void pci_setup(void)
>>           *   the code here assumes it to be.)
>>           * Should either of those two conditions change, this code will 
>> break.
>>           */
>> -        using_64bar = bars[i].is_64bar && bar64_relocate
>> -            && (mmio_total > (mem_resource.max - mem_resource.base));
>> +        using_64bar = bars[i].is_64bar && bar64_relocate &&
>> +            (mmio_total > (mem_resource.max - mem_resource.base) ||
>> +             bar_sz > HUGE_BAR_THRESH);
> 
> There's a comment above this that starts with:
> 
> "Relocate to high memory if the total amount of MMIO needed is more
> than the low MMIO available."
> 
> I would slightly reword it to:
> 
> "Relocate to high memory if the total amount of MMIO needed is more
> than the low MMIO available or BARs bigger that HUGE_BAR_THRESH are
> present."

Hmm, yes, done.

Jan