I would like to propose an interface change to the following
paravirt-ops calls:
void (fastcall *write_ldt_entry)(void *dt, int entrynum, u64 entry);
void (fastcall *write_gdt_entry)(void *dt, int entrynum, u64 entry);
void (fastcall *write_idt_entry)(void *dt, int entrynum, u64 entry);
Can we consolidate the dt and entrynum parameters and just pass
dt+entrynum*8? I don't know if this makes things harder for the Xen
case, but I seem to distantly recall that we used to pass a pure pointer
- actually we used to have a pure post-update call, but changed the
interface to accommodate Xen. When we did that, the interface changed
to take the new descriptor value as a parameter, as a raw write to the
GDT / LDT would not be allowed. And during that step, the new entrynum
parameter was added.
Since it's fairly easy to merely subtract the entry offset from the
descriptor base, and descriptor tables are virtually mapped, obtaining
the page number, if it is required, is still a very easy task. I
believe we added the entrynum field so the page number could be
computed, but I just don't remember why, and don't see any reason for it
now.
So I propose changing the convention here to eliminate the entrynum
field, thus allowing all the parameters to fit within the fastcall
convention, which affords us three registers. The primary motivation
being, the call to write_gdt_entry is on the fast path in context switch.
Any objections?
Zach
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