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    SubjectRe: [PATCH v8 13/16] x86/virt/tdx: Configure global KeyID on all packages
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    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@google.com/T/#m1ff338686cfcb7ba691cd969acc17b32ff194073
    > > https://lore.kernel.org/lkml/de6b69781a6ba1fe65535f48db2677eef3ec6a83.1667110240.git.isaku.yamahata@intel.com/
    > >
    > > 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.
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