Re: Do the x86 kernel entry points need an xabort on TSX cpus?

[Simon Farnsworth]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Andy Lutomirski wrote:

> On Fri, Feb 10, 2012 at 9:18 AM, H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
> wrote:
>> On 02/09/2012 11:40 PM, Andy Lutomirski wrote:
>>>
>>> [...]
>>>
>>> - Ring transitions: SYSENTER, SYSCALL, SYSEXIT, and SYSRET.
>>>
>>> I suspect that many bits of the kernel expect that things they do
>>> won't unhappen.  For example, it could be fun to do:
>>>
>>
>> That's why entering the kernel will cause an abort.  In other words, you
>> will ALWAYS abort when you do a read(), and you will never reach your
>> _xabort().
> 
> Is that architecturally guaranteed?  (My manual suggests that it's
> specifically *not* guaranteed, which is surprising.)
> 
> --Andy
My understanding of the architecture manual's wording (which is a bit 
clumsy) is that they want to leave themselves wiggle room just in case they 
work out a way to do any of these things without requiring an abort.

If, for example, Intel add an MSR for SYSENTER that's used to go to a 
different entrypoint if you're mid-transaction, you've suddenly broken all 
code that assumes that SYSENTER triggers an abort - instead, some SYSENTERs 
trigger an abort (as the kernel does the XABORT), while others don't.

Current implementations appear to always abort on ring transition, though.
- -- 
Simon Farnsworth
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.11 (GNU/Linux)

iQEcBAEBAgAGBQJPNWclAAoJEIKsye9/dtRWNoAH/j6H6mUzk+eS4yeZlcfd1DbG
cIicIJWGJNfm/TWAGb2ABrFgyDS+568ODFUogtAoLFcaxUieVVmuopjfgdjfiLdr
GoANhzyzohknQnHyiQetyTOzmkQVYCrMuRt/qplMO+k5DuvuN0FNxGW990B4jwQL
kFC2KSDMi2QKUnla2XbVsHR7xqe8gRJMEVB5DREkFiVhJGaf4Eyj0Rh4yLfSu9Ka
IngcU7Q6dmSlwCzmt/r+5BJeMvzfDa76+NxdStYDxe2FcZx7BdHeUwM9YyRPcrFp
Cgosn+C8Aiea15Ti/xYpd+M8LWGK8bq4XiV9a8D9WSagGngBrO1u1iCnx2hYKdI=
=hHwE
-----END PGP SIGNATURE-----


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/