Re: [PATCH 01/11] memattrs: add debug attribute

Peter Maydell
2020-12-01T11:57:35Z

On Tue, 1 Dec 2020 at 11:51, Dr. David Alan Gilbert <dgilbert@redhat.com> wrote:
>
> * Peter Maydell (peter.maydell@linaro.org) wrote:
> > On Mon, 16 Nov 2020 at 19:28, Ashish Kalra <Ashish.Kalra@amd.com> wrote:
> > >
> > > From: Brijesh Singh <brijesh.singh@amd.com>
> > >
> > > From: Brijesh Singh <brijesh.singh@amd.com>
> > >
> > > Extend the MemTxAttrs to include a 'debug' flag. The flag can be used as
> > > general indicator that operation was triggered by the debugger.
> > >
> > > A subsequent patch will set the debug=1 when issuing a memory access
> > > from the gdbstub or HMP commands. This is a prerequisite to support
> > > debugging an encrypted guest. When a request with debug=1 is seen, the
> > > encryption APIs will be used to access the guest memory.
> >
> > So, what counts as "debug" here, and why are debug requests
> > special? If "debug=1" means "can actually get at the guest memory",
> > why wouldn't every device model want to use it?
>
> SEV has a flag that the guest-owner can set on a VM to enable debug;
> it's rare for it to be enabled; so it's not suitable for use by normal
> devices. It's only there for debug if the guest owner allows you to.

So if I do a memory transaction with debug=1 then I should expect
that it might come back with a failure status (meaning "this VM
doesn't permit debug") and I should handle that error ?

thanks
-- PMM