Proposed interface change

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

Yeah, write_idt_entry can't be done with just a pointer because we need to get the index out of it, and we don't necessarily know the base of the table being written at that time (since it might not be the main idt_table).

So what Christian said, basically.

J

Do you mean for the TLS update, or something else? There's already a load_tls pv_op.

J

Yes, good point. I don't think it matters so much then.

Zach