Re: [PATCH 10/13] x86/tdx: Convert shared memory back to private on kexec

[Kalra, Ashish]

On 10/6/2023 10:11 AM, Kirill A. Shutemov wrote:
> On Fri, Oct 06, 2023 at 07:58:03AM -0700, Sean Christopherson wrote:
> > On Thu, Oct 05, 2023, Kirill A. Shutemov wrote:
> > > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > > index 7368d254d01f..b5acf9fb4c70 100644
> > > --- a/arch/x86/Kconfig
> > > +++ b/arch/x86/Kconfig
> > > @@ -884,6 +884,7 @@ config INTEL_TDX_GUEST
> > >   	select X86_MEM_ENCRYPT
> > >   	select X86_MCE
> > >   	select UNACCEPTED_MEMORY
> > > +	select EMERGENCY_VIRT_CALLBACK
> > >   	help
> > >   	  Support running as a guest under Intel TDX.  Without this support,
> > >   	  the guest kernel can not boot or run under TDX.
> >
> > ...
> >
> > >   void __init tdx_early_init(void)
> > >   {
> > >   	struct tdx_module_args args = {
> > > @@ -882,6 +1007,14 @@ void __init tdx_early_init(void)
> > >   	 */
> > >   	x86_cpuinit.parallel_bringup = false;
> > >   
> > > +	machine_ops.shutdown = tdx_shutdown;
> > > +
> > > +	/*
> > > +	 * KVM overrides machine_ops.crash_shutdown, use emergency
> >
> > This is going to be super confusing.  KVM utilizes the emergency virt callback.
> > The KVM paravirt guest code uses .crash_shutdown().  People that are passingly
> > familiar with virt and know what KVM is, but don't already know the difference
> > between the two are going to be all kinds of confused.
> >
> > I also feel like you're playing with fire, e.g. what's to stop the hypervisor
> > specific paravirt guest support from using .shutdown() in the future?
> >
> > And the callback is invoked for far more than just kexec().  I don't see how the
> > host can emulate a reboot without destroying and rebuilding the VM, e.g. it can't
> > stuff register state to emulate INIT or RESET.  Unless I'm missing something,
> > converting shared memory back to private for a shutdown or reboot is undesirable
> > as adds one more thing that can go wrong and prevent the system from cleanly
> > shutting down ASAP (for some definitions of "cleanly").
>
> Okay, fair enough. I will look for better way to hookup into kexec
> process. That was the best fit I found so far, but yes it is not ideal.
>
> > Lastly, doesn't SEV need similar behavior?  This seems like core functionality
> > for any guest with cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT).  Why not make the
> > "unshare on kexec" code common and gate it with CC_ATTR_GUEST_MEM_ENCRYPT?
>
> I don't know SEV specifics. I am open to collaboration on this.
>
> Tom, Ashish, let me know if you need this in generic code. I can arrange
> that.

Yes, some kind of a generic interface like unshare_on_kexec() gated with 
CC_ATTR_GUEST_MEM_ENCRYPT is needed, we can then add SNP specific kexec 
functionality as part of this.

Thanks,
Ashish