[Xen-devel] portability issues

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

Being in the process of trying to enable sysenter/syscall use from compat mode
guests and compat mode apps in native guests (for performance to a certain
degree, but more importantly - at least for the compat mode app case - to get
closer to native behavior, i.e. mid/long term requiring less kernel 
modifications)
I'm facing a few backwards compatibility issues that I'm not really sure how to
deal with:

1) Obviously I need to extend the guest_context structure (to store the
additional callback addresses), but there do not seem to be provisions to do
so without breaking the dom0 interface. I'm currently considering adding a
flag indicating use of the larger structure, but this certainly doesn't scale 
well
considering future additions. An alternative might be to add a single flag
covering all future additions, and using the first field past the current size 
to
store the overall or add-on size, so that the hypervisor has a way to know
how much of the structure to copy.

2) While the x86-32 hv can't support syscall and is unlikely to support 
sysenter,
save/restore/migration (which hopefully will work at least from 32-bit hv to
64-bit hv in the future) imposes an issue here in that native wouldn't need
these extra fields, but a compat mode guest would have to have a way to
store them (in compat mode format), implying that guest_context would also
need to be extended for 32-bits.

3) Currently, the 32-bit kernel check X86_FEATURE_SEP and the return
status of setting the sysenter hypercall, to detect its availability when run
in supervisor mode. This, however, is being done only on the boot CPU,
which works thanks to a quirk in how Xen handles the hypercall - MSRs for
all CPUs get set by this single call, which clearly doesn't match native
behavior (where in theory all CPUs could have distinct settings and have
to establish them as they come up). The problem with this is that if things
turn out to work as intended, X86_FEATURE_SEP will be seen enabled by
the guest when run on a 64-bit hv, and setting the sysenter callback will
also succeed, but existing guests will fail to set the callback on all CPUs.
I have to admit that I'm rather reluctant to add the same kind of quirk for
!supervisor_mode_kernel in the hypervisor's handling of CALLBACK_sysenter.

Thanks for suggestions/opinions,
Jan


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