Re: [PATCH v8 13/16] x86/virt/tdx: Configure global KeyID on all packages

[Huang, Kai]

On Tue, 2023-01-10 at 08:53 -0800, Dave Hansen wrote:
> On 1/10/23 02:15, Huang, Kai wrote:
> > On Fri, 2023-01-06 at 14:49 -0800, Dave Hansen wrote:
> > > On 12/8/22 22:52, Kai Huang wrote:
> ...
> > > > + * Note:
> > > > + *
> > > > + * This function neither checks whether there's at least one online cpu
> > > > + * for each package, nor explicitly prevents any cpu from going offline.
> > > > + * If any package doesn't have any online cpu then the SEAMCALL won't be
> > > > + * done on that package and the later step of TDX module initialization
> > > > + * will fail.  The caller needs to guarantee this.
> > > > + */
> > > 
> > > *Does* the caller guarantee it?
> > > 
> > > You're basically saying, "this code needs $FOO to work", but you're not
> > > saying who *provides* $FOO.
> > 
> > In short, KVM can do something to guarantee but won't 100% guarantee this.
> > 
> > Specifically, KVM won't actively try to bring up cpu to guarantee this if
> > there's any package has no online cpu at all (see the first lore link below).
> > But KVM can _check_ whether this condition has been met before calling
> > tdx_init() and speak out if not.  At the meantime, if the condition is met,
> > refuse to offline the last cpu for each package (or any cpu) during module
> > initialization.
> > 
> > And KVM needs similar handling anyway.  The reason is not only configuring the
> > global KeyID has such requirement, creating/destroying TD (which involves
> > programming/reclaiming one TDX KeyID) also require at least one online cpu for
> > each package.
> > 
> > There were discussions around this on KVM how to handle.  IIUC the solution is
> > KVM will:
> > 1) fail to create TD if any package has no online cpu.
> > 2) refuse to offline the last cpu for each package when there's any _active_ TDX
> > guest running.
> > 
> > https://lore.kernel.org/lkml/20221102231911.3107438-1-seanjc@xxxxxxxxxx/T/#m1ff338686cfcb7ba691cd969acc17b32ff194073
> > https://lore.kernel.org/lkml/de6b69781a6ba1fe65535f48db2677eef3ec6a83.1667110240.git.isaku.yamahata@xxxxxxxxx/
> > 
> > Thus TDX module initialization in KVM can be handled in similar way.
> > 
> > Btw, in v7 (which has per-lp init requirement on all cpus), tdx_init() does
> > early check on whether all machine boot-time present cpu are online and simply
> > returns error if condition is not met.  Here the difference is we don't have any
> > check but depend on SEAMCALL to fail.  To me there's no fundamental difference.
> 
> So, I'm going to call shenanigans here.
> 
> You say:
> 
> 	The caller needs to guarantee this.
> 
> Then, you go and tell us how the *ONE* caller of this function doesn't
> actually guarantee this.  Plus, you *KNOW* this.
> 
> Those are shenanigans.

Agreed.

> 
> Let's do something like this instead of asking for something impossible
> and pretending that the callers are going to provide some fantasy solution.
> 
> /*
>  * Attempt to configure the global KeyID on all physical packages.
>  *
>  * This requires running code on at least one CPU in each package.  If a
>  * package has no online CPUs, that code will not run and TDX module
>  * initialization (TDH.whatever) will fail.
>  *
>  * This code takes no affirmative steps to online CPUs.  Callers (aka.
>  * KVM) can ensure success by ensuring sufficient CPUs are online for
>  * this to succeed.
>  */

Thanks.  Will update changelog accordingly.

> 
> Now, since this _is_ all imperfect, what will our users see if this
> house of cards falls down?  Will they get a nice error message like:
> 
>      TDX: failed to configure module, no online CPUs in package 12
> 
> Or, will they see:
> 
>      TDX: Hurr, durr, I'm confused and you should be too
> 
> ?

I am expecting the former.  I will work with Isaku to make sure of it.