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

From: Zachary Amsden
Date: Mon Aug 07 2006 - 16:49:30 EST

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).

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