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Name: Stephan Keil
eMail: Stephan.Keil@gmx.de
Date: June 10, 2003 at 09:09:36
Subject: Re: Hyperthreading and 'lock' prefix
In Reply To: Re: Hyperthreading and 'lock' prefix by
james on June 10, 2003 at 04:01:12
Text: || the x86 instruction set supports the 'lock' prefix for various
|| instructions (e.g. inc, add) to guarantee atomicity in multiprocessor
|| environments, i.e. multiple _physical_ processors. But is this
|| prefix also necessary for hyperthreading processors, i.e. two
|| logical processors within one physical processor?
|
| Yes.
|
| I mean, how else is virtual processor #2 gonna know
| that it shouldn't write to a location that virtual
| processor #1 is currently operating on with a read-
| modify-write operation?

Both processors share the same pipeline. If the pipeline would care
about read-modify-write conflicts regardless of the executing
thread (as it does for a single thread), then it would be safe to omit
the 'lock'. So my question refers more to the current implementations
of hyperthreaded processors (P4-XEON?!): does the P4 pipeline care about
those conflicts regardless of the executing thread?


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