Re: [PATCH v2 1/4] xl: Add support for ignore_msrs option

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Jan 20, 2021 at 05:49:09PM -0500, Boris Ostrovsky wrote:
> This option allows guest administrator specify what should happen when
> guest accesses an MSR which is not explicitly emulated by the hypervisor.
> 
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> ---
>  docs/man/xl.cfg.5.pod.in         | 20 +++++++++++++++++++-
>  tools/libs/light/libxl_types.idl |  7 +++++++
>  tools/xl/xl_parse.c              |  7 +++++++
>  3 files changed, 33 insertions(+), 1 deletion(-)
> 
> diff --git a/docs/man/xl.cfg.5.pod.in b/docs/man/xl.cfg.5.pod.in
> index c8e017f950de..96ce97c42cab 100644
> --- a/docs/man/xl.cfg.5.pod.in
> +++ b/docs/man/xl.cfg.5.pod.in
> @@ -2044,7 +2044,25 @@ Do not provide a VM generation ID.
>  See also "Virtual Machine Generation ID" by Microsoft:
>  
> L<https://docs.microsoft.com/en-us/windows/win32/hyperv_v2/virtual-machine-generation-identifier>
>  
> -=back 
> +=over
> +
> +=item B<ignore_msrs="STRING">
> +
> +Determine hypervisor behavior on accesses to MSRs that are not emulated by 
> the hypervisor.
> +
> +=over 4
> +
> +=item B<never>
> +
> +Issue a warning to the log and #GP to the guest. This is default.
> +
> +=item B<silent>
> +
> +MSR reads return 0, MSR writes are ignored. No warnings to the log.
> +
> +=item B<verbose>
> +
> +Similar to B<silent> but a warning is written.

Would it make sense to allow for this option to be more fine-grained
in the future?

Not that you need to implement the full thing now, but maybe we could
have something like:

"
=item B<ignore_msrs=[ "MSR_RANGE, "MSR_RANGE", ..]>

Specify a list of MSR ranges that will be ignored by the hypervisor:
reads will return zeros and writes will be discarded without raising a
#GP.

Each MSR_RANGE is given in hexadecimal format and may be a range, e.g.
c00102f0-c00102f1 (inclusive), or a single MSR, e.g. c00102f1.
"

Then you can print the messages in the hypervisor using a guest log
level and modify it on demand in order to get more verbose output?

I don't think selecting whether the messages are printed or not from
xl is that helpful as the same could be achieved using guest_loglvl.

Also I think it will be fine to only implement:

ignore_msrs=[ "0-ffffffff" ]

Right now and return an error for any other combination, so that we
can get something in soon and expand it later.

Thanks, Roger.