Re: [PATCH 1/4] x86 paravirt_ops: create no_paravirt.h for native ops

Zachary Amsden
2006-08-07T20:51:52Z

Rusty Russell wrote:
>>> +
>>> +/*
>>> + * Set IOPL bits in EFLAGS from given mask
>>> + */
>>> +static inline void set_iopl_mask(unsigned mask)
>>>
>> This function can be completely written in C using local_save_flags()/local_restore_flags()
>> Please do that. I guess it's still a good idea to keep it separated
>> though because it might allow other optimizations.
>>
>> e.g. i've been thinking about special casing IF changes in save/restore flags
>> to optimize CPUs which have slow pushf/popf. If you already make sure
>> all non IF manipulations of flags are separated that would help.
>>

Actually, that is not quite true. Local_save_flags /
raw_local_irq_restore today is used only for operating on IF flag, and
raw_local_restore_flags does not exist. Our implementation of these in
VMI assumes that only the IF flag is being changed, and this is the
default assumption under which Xen runs as well. Using local_restore to
switch IOPL as well causes the extremely performance critical common
case of pure IRQ restore to do potentially a lot more work in a hypervisor.

So if you do want us to go with the C approach, I would propose using
raw_local_iopl_restore, which can make a different hypercall (actually,
in our case, this is not even a hypercall, merely a VMI call).

Zach