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

[kirill.shutemov@xxxxxxxxxxxxxxx]

On Mon, Jun 12, 2023 at 03:06:48AM +0000, Huang, Kai wrote:
> 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();

If you want to play with barriers, assign pamt_4k_base the last with
smp_store_release() and read it first in tdmr_get_pamt() with
smp_load_acquire(). If it is non-zero, all pamt_* fields are valid.

Or just drop this non-sense and use a spin lock for serialization.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov