Re: [Xen-devel] [PATCH v4 3/3] xen/efi: use directmap to access runtime services table

["marmarek@xxxxxxxxxxxxxxxxxxxxxx" <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Thu, Oct 24, 2019 at 01:11:22PM +0000, Xia, Hongyan wrote:
> It is certainly nice to have less users of the direct map. My non-EFI
> builds already work without the direct map now but once I start testing
> EFI, it is nice to have one less thing to worry about.

Note this is just yet another EFI info that's included there. Others
are: efi_ct, efi_memmap, efi_fw_vendor. So, if you'd like to get rid of
directmap, you'll need to handle the others too in some way. Doing that
for 3 or 4 tables shouldn't make significant difference.

> How important and performance-critical is this? If we really want to
> avoid switching the page table, we could reserve a virtual range and
> map it to runtime services in Xen.

Honestly I don't think that's very critical. The biggest improvement is
for XEN_FW_EFI_RT_VERSION, where you avoid switching page tables at all.
In other cases, you avoid that for too old UEFIs only. Anyway, I think
none of it is on a hot path.
This is an optimization suggested by Jan, which is nice to have, but
definitely isn't the only possible option.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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