Re: [PATCH v13 20/22] x86/kexec(): Reset TDX private memory on platforms with TDX erratum

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

> > > 
> > 
> > Good point.  Thanks!
> > 
> > Based on my understanding, it should be OK to skip tdx_reset_memory()
> > (or better
> > to) when preserve_context is on.  The second kernel shouldn't touch
> > first
> > kernel's memory anyway otherwise it may corrupt the first kernel
> > state (if it
> > does this maliciously or accidentally, then the first kernel isn't
> > guaranteed to
> > work anyway).  
> 
> I think it may read the memory, is it ok?

Read is fine.  Only "partial write" can poison the memory.

[...]
> 

> 
> Not the most beautiful ifdeffery, I'd just duplicate the
> tdx_reset_memory() call. But not a strong opinion.
> 

Refined to below.  Let me know if you have any further comments?

--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -307,12 +307,19 @@ void machine_kexec(struct kimage *image)
         * all TDX private pages need to be converted back to normal
         * before booting to the new kernel, otherwise the new kernel
         * may get unexpected machine check.
+        *
+        * But skip this when preserve_context is on.  The second kernel
+        * shouldn't write to the first kernel's memory anyway.  Skipping
+        * this also avoids killing TDX in the first kernel, which would
+        * require more complicated handling.
         */
-       tdx_reset_memory();
-
 #ifdef CONFIG_KEXEC_JUMP
        if (image->preserve_context)
                save_processor_state();
+       else
+               tdx_reset_memory();
+#else
+       tdx_reset_memory();
 #endif