Re: [PATCH v2] powerpc: Ignore DSI error caused by the copy/paste instruction

[Haren Myneni <haren@linux.ibm.com>]

On Mon, 2022-09-26 at 05:55 +0000, Christophe Leroy wrote:
> 
> Le 25/09/2022 à 22:26, Haren Myneni a écrit :
> > DSI error will be generated when the paste operation is issued on
> > the suspended NX window due to NX state changes. The hypervisor
> > expects the partition to ignore this error during page pault
> > handling. To differentiate DSI caused by an actual HW configuration
> > or by the NX window, a new “ibm,pi-features” type value is defined.
> > Byte 0, bit 3 of pi-attribute-specifier-type is now defined to
> > indicate this DSI error. If this error is not ignored, the user
> > space can get SIGBUS when the NX request is issued.
> 
> Would be nice to mention at least one time in the message that NX
> stands 
> to nest accelerator.
> 
> Otherwise, that's confusing with for exemple:
> Commit 2e602847d9c2 ("KVM: PPC: Don't flush PTEs on NX/RO hit")
> Commit c49643319715 ("powerpc/32s: Only leave NX unset on segments
> used 
> for modules")

Thanks. I did not realize since VAS/NX code is added before. I will add
the description as you suggested. 

> 
> 
> > This patch adds changes to read ibm,pi-features property and ignore
> > DSI error in the page fault handling if CPU_FTR_NX_DSI if defined.
> > 
> > Signed-off-by: Haren Myneni <haren@linux.ibm.com>
> > ---
> > v2: Code cleanup as suggested by Christophe Leroy
> > 
> >   arch/powerpc/include/asm/cputable.h |  5 ++--
> >   arch/powerpc/kernel/prom.c          | 36 +++++++++++++++++++++---
> > -----
> >   arch/powerpc/mm/fault.c             | 17 +++++++++++++-
> >   3 files changed, 45 insertions(+), 13 deletions(-)
> > 
> > diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> > index 014005428687..cb949f12baa9 100644
> > --- a/arch/powerpc/mm/fault.c
> > +++ b/arch/powerpc/mm/fault.c
> > @@ -367,7 +367,22 @@ static void sanity_check_fault(bool is_write,
> > bool is_user,
> >   #elif defined(CONFIG_PPC_8xx)
> >   #define page_fault_is_bad(__err)	((__err) & DSISR_NOEXEC_OR_G)
> >   #elif defined(CONFIG_PPC64)
> > -#define page_fault_is_bad(__err)	((__err) & DSISR_BAD_FAULT_64S)
> > +static int page_fault_is_bad(unsigned long err)
> > +{
> > +	unsigned long flag = DSISR_BAD_FAULT_64S;
> > +
> > +	/*
> > +	 * PAPR 14.15.3.4.1
> > +	 * If byte 0, bit 3 of pi-attribute-specifier-type in
> > +	 * ibm,pi-features property is defined, ignore the DSI error
> > +	 * which is caused by the paste instruction on the
> > +	 * suspended NX window.
> > +	 */
> > +	if (cpu_has_feature(CPU_FTR_NX_DSI))
> > +		flag &= ~DSISR_BAD_COPYPASTE;
> > +
> > +	return (err & flag);
> 
> You don't need parenthesis ( )
> 
> > +}
> >   #else
> >   #define page_fault_is_bad(__err)	((__err) & DSISR_BAD_FAULT_32S)
> >   #endif