[PATCH] drm/ttm: set TTM allocated pages as reserved

[Paolo Bonzini]

On 3/29/23 17:29, Christian König wrote:
>> First, is it a _host_ corruption or a guest corruption/crash?  A guest 
>> crash would be KVM doing exactly what it's meant to do: it detects the 
>> non-reserved, non-refcounted page and refuses to map it into the guest.
> 
> Yes I think that this is what happens.

Ok I was worried all the time that this was host corruption/crash; which 
obviously would have been much worse.

> The use case and mapping is indeed valid as far as I can see, but
> the handling that KVM does here is really problematic.
> 
> What needs to happen instead is that when both hva_to_pfn_fast() and 
> hva_to_pfn_slow() fails you don't try to convert the PFN into a page and 
> so also don't get a reference to the page.
> 
> This somehow needs to be communicated to the callers of hva_to_pfn() so 
> that kvm_release_pfn() knows what to do.

There's a bit more complication here:

1) in the guest page fault path we can avoid taking the reference altogether

2) in other MMU-notifier-protected paths, we can also avoid taking the 
reference but we also must stop using kmap() in virt/kvm/pfncache.c.

3) other uses of kmap() must switch to MMU-notifier protection.


If the DRM people are okay with SetPageReserved() as a temporary hack, 
we can change or remove the WARN in kvm_is_zone_device_page(), since 
that is what you are referring to in the commit message.

Paolo