Re: [Xen-devel] [PATCH v3] xen/balloon: Mark unallocated host memory as UNUSABLE

Boris Ostrovsky
2018-11-26T19:42:40Z

On 11/26/18 12:10 PM, Igor Druzhinin wrote:
> On 26/11/2018 16:25, Boris Ostrovsky wrote:
>> On 11/25/18 8:00 PM, Igor Druzhinin wrote:
>>> On 20/12/2017 14:05, Boris Ostrovsky wrote:
>>>> Commit f5775e0b6116 ("x86/xen: discard RAM regions above the maximum
>>>> reservation") left host memory not assigned to dom0 as available for
>>>> memory hotplug.
>>>>
>>>> Unfortunately this also meant that those regions could be used by
>>>> others. Specifically, commit fa564ad96366 ("x86/PCI: Enable a 64bit BAR
>>>> on AMD Family 15h (Models 00-1f, 30-3f, 60-7f)") may try to map those
>>>> addresses as MMIO.
>>>>
>>>> To prevent this mark unallocated host memory as E820_TYPE_UNUSABLE (thus
>>>> effectively reverting f5775e0b6116) and keep track of that region as
>>>> a hostmem resource that can be used for the hotplug.
>>>>
>>>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>>> This commit breaks Xen balloon memory hotplug for us in Dom0 with
>>> "hoplug_unpopulated" set to 1. The issue is that the common kernel
>>> memory onlining procedures require "System RAM" resource to be 1-st
>>> level. That means by inserting it under "Unusable memory" as the commit
>>> above does (intentionally or not) we make it 2-nd level and break memory
>>> onlining.
>> What do you mean by 1st and 2nd level?
>>
> I mean the level of a resource in IOMEM tree (the one that's printed
> from /proc/iomem). 1-st level means its parent is root and so on.

Ah, OK. Doesn't
additional_memory_resource()->insert_resource(iomem_resource) place the
RAM at 1st level? And if not, can we make it so?

>
>>> There are multiple ways to fix it depending on what was the intention of
>>> original commit and what exactly it tried to workaround. It seems it
>>> does several things at once:
>>> 1) Marks non-Dom0 host memory "Unusable memory" in resource tree.
>>> 2) Keeps track of all the areas safe for hotplug in Dom0
>>> 3) Changes allocation algorithms itself in balloon driver to use those areas
>> Pretty much. (3) is true in the sense that memory is first allocated
>> from hostmem_resource (which is non-dom0 RAM).
>>
>>> Are all the things above necessary to cover the issue in fa564ad96366
>>> ("x86/PCI: Enable a 64bit BAR on AMD Family 15h (Models 00-1f, 30-3f,
>>> 60-7f)")?
>> Not anymore, as far as that particular commit is concerned, but that's
>> because of 03a551734 ("x86/PCI: Move and shrink AMD 64-bit window to
>> avoid conflict") which was introduced after balloon patch. IIRC there
>> were some issues with fa564ad96366 unrelated to balloon.
>>
> If it's not a problem anymore IIUC, can we revert the change as it still
> breaks "hotplug_unpopulated=1" for the reasons I described above?

Since this seems to have broken existing feature this would be an
option. But before going that route I'd like to see if we can fix the patch.

I have been unable to reproduce your problem. Can you describe what you did?

>
>>> Can we remove "Unusable memory" resources as soon as we finished
>>> booting? Is removing on-demand is preferable over "shoot them all" in
>>> that case?
>> The concern is that in principle nothing prevents someone else to do
>> exact same thing fa564ad96366 did, which is grab something from right
>> above end of RAM as the kernel sees it. And that can be done at any point.
>>
> Nothing prevents - true, but that's plainly wrong from OS point of view
> to grab physical ranges for something without knowing what's actually
> behind on that platform.

I am not sure I agree that this is plainly wrong. If not for BIOS issues
that 03a551734cf mentions I think what the original implementation of
fa564ad963 did was perfectly reasonable. Which is why I would prefer to
keep keep the hostmem resource *if possible*.

-boris

> I think we shouldn't consider this as a valid
> thing to do and don't try to workaround initially incorrect code.
>
>> -boris
>>
>>> Does it even make sense to remove the 1-st level only restriction in
>>> kernel/resource.c ?
>>
>>