Re: [Xen-devel] RE: [Xen-changelog] [xen-unstable] Initial support for HVM compat guests

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 15/1/07 8:07 am, "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:

> So is this still on development for HVM? I didn't find where
> _DOMF_COMPAT is set for 32bit HVM guest, and then compat
> logic under is_hvm_vcpu may not make effect yet. (Correct me if
> I'm wrong)

An HVM guest does not use DOMF_COMPAT. You can deduce its execution mode
from CS attribute bytes. And of course it can switch execution mode back and
forth as it runs, and on different VCPUs, etc.

> Other incompleteness is like, XLAT_cpu_user_regs is invoked
> after hvm_store_cpu_guest_regs in arch_get_info_guest, while
> arch_set_info_guest invokes hvm_load_cpu_guest_regs directly,
> etc.

Yes, that's an incompleteness. Stuff is getting implemented on demand. Noone
uses arch_set_info_guest() on an HVM guest, but xc_ptrace() does use
arch_get_info_guest() so I guess Jan fixed that one up.

> Also, one question is whether COMPAT mode requires all guests
> to be in compatible, or can be mixed like 64bit dom0 + 32bit domU.
> Still take arch_get_info_guest for example:
>    Vcpu_guest_context is translated if target domain is compat
> mode, while there's no check for current domain's mode. Then
> compat context is copied back to dom0 even when dom0 is a 64bit
> guest.

Currently we require all guests to be in the same mode. However, there are
tools changes in progress to allow mixed-mode guests. Checking COMPAT of
caller vs. target is indeed a bit confusing. We'll audit the domctl
interfaces and make up our mind how that should work when we apply the tools
changes for mixed-mode guests.

> Are things like above deliberately made to depend on dom0 aware
> of target guest context layout, or something still immature? I just want
> to get a clear picture what impact this new compatible mode may bring
> to the whole environment. :-)

dom0 must be aware of target guest context. It builds the page tables for
example. :-) So it is acceptable for him to have to use different domctl()
commands to initialise 32- vs 64-bit pv guests (although we obviously want
to avoid bloating the interface with compat crap as far as possible).

 -- Keir



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