Re: [PATCH v11 18/20] x86: Handle TDX erratum to reset TDX private memory during kexec() and reboot

[Huang, Kai]

On Fri, 2023-06-09 at 16:23 +0300, kirill.shutemov@xxxxxxxxxxxxxxx wrote:
> On Mon, Jun 05, 2023 at 02:27:31AM +1200, Kai Huang wrote:
> > diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> > index 8ff07256a515..0aa413b712e8 100644
> > --- a/arch/x86/virt/vmx/tdx/tdx.c
> > +++ b/arch/x86/virt/vmx/tdx/tdx.c
> > @@ -587,6 +587,14 @@ static int tdmr_set_up_pamt(struct tdmr_info *tdmr,
> >  		tdmr_pamt_base += pamt_size[pgsz];
> >  	}
> >  
> > +	/*
> > +	 * tdx_memory_shutdown() also reads TDMR's PAMT during
> > +	 * kexec() or reboot, which could happen at anytime, even
> > +	 * during this particular code.  Make sure pamt_4k_base
> > +	 * is firstly set otherwise tdx_memory_shutdown() may
> > +	 * get an invalid PAMT base when it sees a valid number
> > +	 * of PAMT pages.
> > +	 */
> 
> Hmm? What prevents compiler from messing this up. It can reorder as it
> wishes, no?

Hmm.. Right. Sorry I missed.

> 
> Maybe add a proper locking? Anything that prevent preemption would do,
> right?
> 
> >  	tdmr->pamt_4k_base = pamt_base[TDX_PS_4K];
> >  	tdmr->pamt_4k_size = pamt_size[TDX_PS_4K];
> >  	tdmr->pamt_2m_base = pamt_base[TDX_PS_2M];
> 

I think a simple memory barrier will do.  How does below look?

--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -591,11 +591,12 @@ static int tdmr_set_up_pamt(struct tdmr_info *tdmr,
         * tdx_memory_shutdown() also reads TDMR's PAMT during
         * kexec() or reboot, which could happen at anytime, even
         * during this particular code.  Make sure pamt_4k_base
-        * is firstly set otherwise tdx_memory_shutdown() may
-        * get an invalid PAMT base when it sees a valid number
-        * of PAMT pages.
+        * is firstly set and place a __mb() after it otherwise
+        * tdx_memory_shutdown() may get an invalid PAMT base
+        * when it sees a valid number of PAMT pages.
         */
        tdmr->pamt_4k_base = pamt_base[TDX_PS_4K];
+       __mb();