Re: [PATCH v11 15/20] x86/virt/tdx: Configure global KeyID on all packages

[kirill . shutemov]

On Mon, Jun 05, 2023 at 02:27:28AM +1200, Kai Huang wrote:
> After the list of TDMRs and the global KeyID are configured to the TDX
> module, the kernel needs to configure the key of the global KeyID on all
> packages using TDH.SYS.KEY.CONFIG.
> 
> This SEAMCALL cannot run parallel on different cpus.  Loop all online
> cpus and use smp_call_on_cpu() to call this SEAMCALL on the first cpu of
> each package.
> 
> To keep things simple, this implementation takes no affirmative steps to
> online cpus to make sure there's at least one cpu for each package.  The
> callers (aka. KVM) can ensure success by ensuring that.
> 
> Intel hardware doesn't guarantee cache coherency across different
> KeyIDs.  The PAMTs are transitioning from being used by the kernel
> mapping (KeyId 0) to the TDX module's "global KeyID" mapping.
> 
> This means that the kernel must flush any dirty KeyID-0 PAMT cachelines
> before the TDX module uses the global KeyID to access the PAMTs.
> Otherwise, if those dirty cachelines were written back, they would
> corrupt the TDX module's metadata.  Aside: This corruption would be
> detected by the memory integrity hardware on the next read of the memory
> with the global KeyID.  The result would likely be fatal to the system
> but would not impact TDX security.
> 
> Following the TDX module specification, flush cache before configuring
> the global KeyID on all packages.  Given the PAMT size can be large
> (~1/256th of system RAM), just use WBINVD on all CPUs to flush.
> 
> If TDH.SYS.KEY.CONFIG fails, the TDX module may already have used the
> global KeyID to write the PAMTs.  Therefore, use WBINVD to flush cache
> before returning the PAMTs back to the kernel.  Also convert all PAMTs
> back to normal by using MOVDIR64B as suggested by the TDX module spec,
> although on the platform without the "partial write machine check"
> erratum it's OK to leave PAMTs as is.
> 
> Signed-off-by: Kai Huang <kai.huang@xxxxxxxxx>
> Reviewed-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>

Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov