Re: [PATCH v6 2/7] x86/sme: Use percpu boolean to control WBINVD during kexec

["Huang, Kai" <kai.huang@intel.com>]

On Tue, 2025-08-19 at 21:28 +0200, Borislav Petkov wrote:
> On Thu, Aug 14, 2025 at 11:59:02AM +1200, Kai Huang wrote:
> > TL;DR:
> > 
> > Prepare to unify how TDX and SME do cache flushing during kexec by
> > making a percpu boolean control whether to do the WBINVD.
> > 
> > -- Background --
> > 
> > On SME platforms, dirty cacheline aliases with and without encryption
> > bit can coexist, and the CPU can flush them back to memory in random
> > order.  During kexec, the caches must be flushed before jumping to the
> > new kernel otherwise the dirty cachelines could silently corrupt the
> > memory used by the new kernel due to different encryption property.
> > 
> > TDX also needs a cache flush during kexec for the same reason.  It would
> > be good to have a generic way to flush the cache instead of scattering
> > checks for each feature all around.
> > 
> > When SME is enabled, the kernel basically encrypts all memory including
> > the kernel itself and a simple memory write from the kernel could dirty
> > cachelines.  Currently, the kernel uses WBINVD to flush the cache for
> > SME during kexec in two places:
> > 
> > 1) the one in stop_this_cpu() for all remote CPUs when the kexec-ing CPU
> >    stops them;
> > 2) the one in the relocate_kernel() where the kexec-ing CPU jumps to the
> >    new kernel.
> > 
> > -- Solution --
> > 
> > Unlike SME, TDX can only dirty cachelines when it is used (i.e., when
> > SEAMCALLs are performed).  Since there are no more SEAMCALLs after the
> > aforementioned WBINVDs, leverage this for TDX.
> > 
> > To unify the approach for SME and TDX, use a percpu boolean to indicate
> > the cache may be in an incoherent state and needs flushing during kexec,
> > and set the boolean for SME.  TDX can then leverage it.
> > 
> > While SME could use a global flag (since it's enabled at early boot and
> > enabled on all CPUs), the percpu flag fits TDX better:
> > 
> > The percpu flag can be set when a CPU makes a SEAMCALL, and cleared when
> > another WBINVD on the CPU obviates the need for a kexec-time WBINVD.
> > Saving kexec-time WBINVD is valuable, because there is an existing
> > race[*] where kexec could proceed while another CPU is active.  WBINVD
> > could make this race worse, so it's worth skipping it when possible.
> > 
> > -- Side effect to SME --
> > 
> > Today the first WBINVD in the stop_this_cpu() is performed when SME is
> > *supported* by the platform, and the second WBINVD is done in
> > relocate_kernel() when SME is *activated* by the kernel.  Make things
> > simple by changing to do the second WBINVD when the platform supports
> > SME.  This allows the kernel to simply turn on this percpu boolean when
> > bringing up a CPU by checking whether the platform supports SME.
> > 
> > No other functional change intended.
> > 
> > [*] The aforementioned race:
> > 
> > During kexec native_stop_other_cpus() is called to stop all remote CPUs
> > before jumping to the new kernel.  native_stop_other_cpus() firstly
> > sends normal REBOOT vector IPIs to stop remote CPUs and waits them to
> > stop.  If that times out, it sends NMI to stop the CPUs that are still
> > alive.  The race happens when native_stop_other_cpus() has to send NMIs
> > and could potentially result in the system hang (for more information
> > please see [1]).
> 
> This text is meandering a bit too much across a bunch of things and could be
> made tighter... Just a nitpick anyway...

Yeah agreed.  I've worked to improve it but ... :-)

I'll keep this in mind and do better in the future!

> 
> >  arch/x86/include/asm/kexec.h         |  4 ++--
> >  arch/x86/include/asm/processor.h     |  2 ++
> >  arch/x86/kernel/cpu/amd.c            | 17 +++++++++++++++++
> >  arch/x86/kernel/machine_kexec_64.c   | 14 ++++++++++----
> >  arch/x86/kernel/process.c            | 24 +++++++++++-------------
> >  arch/x86/kernel/relocate_kernel_64.S | 13 ++++++++++---
> >  6 files changed, 52 insertions(+), 22 deletions(-)
> 
> Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>

Thanks!