Re: [PATCH v2 5/7] perf/amd/ibs: Enable RIP bit63 hardware filtering

[Ravi Bangoria]

Hi Ian,

>> IBS on future hardware adds the ability to filter IBS events by examining
>> RIP bit 63. Because Linux kernel addresses always have bit 63 set while
>> user-space addresses never do, this capability can be used as a privilege
>> filter.
> 
> Since x86's top-byte-ignore/linear-address-masking leaves bit 63 could
> this break in the future if the kernel later ignores all bits,
> including bit 63, and user space wants to use bit 63 of the pointer
> for metadata?

The AMD equivalent feature is called Upper Address Ignore (UAI).

o Identifying whether an address is in kernel or user space by examining
  bit 63 is so fundamental in Linux that the UAI design was revisited in
  UAIv2 to restore bit 63 as canonical:
  https://lore.kernel.org/lkml/6a5076ad-405e-4e5e-af55-fe2a6b01467d@xxxxxxxxxxxxxxxx

o UAI applies only to data addresses; instruction addresses must remain
  canonical.

So I assume this should not be an issue, at least for now.

> Does the bit 63 assumption hold for guest operating systems?

Yes, this seems to be an issue, even with current swfilt approach. Let
me inspect the code and get back.

Thanks for the review,
Ravi