Messages in this thread | | | Date | Wed, 25 Mar 2015 10:28:45 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH] x86: vdso32/syscall.S: do not load __USER32_DS to %ss |
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* Andy Lutomirski <luto@amacapital.net> wrote:
> Now we can do a fun hack on top. On Intel, we have > sysenter/sysexitl and, on AMD, we have syscall/sysretl. But, if I > read the docs right, Intel has sysretl, too. So we can ditch > sysexit entirely, since this mechanism no longer has any need to > keep the entry and exit conventions matching.
So this only affects 32-bit vdsos, because on 64-bit both Intel and AMD have and use SYSCALL/SYSRET.
So my question would be: what's the performance difference between INT80 and sysenter entries on 32-bit, on modern CPUs?
If it's not too horrible (say below 100 cycles) then we could say that we start out the simplification and robustification by switching Intel over to INT80 + SYSRET on 32-bit, and once we know the 32-bit SYSRET and all the other simplifications work fine we implement the SYSENTER-hack on top of that?
Is there any user-space code that relies on being able to execute an open coded SYSENTER, or are we shielded via the vDSO?
Doing it this way would make it a lot more practical to pull off, as the incentive to implement the SYSENTER hack on Intel CPUs will be significant: dozens of cycles on 32-bit. Also, I have no problem with putting some pressure on Intel developers, for the absolutely indefensible horror interface that SYSENTER turned out to be! ;-)
Thanks,
Ingo
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