Re: Regression when booting 5.15 as dom0 on arm64 (WAS: Re: [linux-linus test] 161829: regressions - FAIL)]

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 11 May 2021, Christoph Hellwig wrote:
> On Mon, May 10, 2021 at 06:46:34PM -0700, Stefano Stabellini wrote:
> > On Mon, 10 May 2021, Christoph Hellwig wrote:
> > > On Sat, May 08, 2021 at 12:32:37AM +0100, Julien Grall wrote:
> > > > The pointer dereferenced seems to suggest that the swiotlb hasn't been 
> > > > allocated. From what I can tell, this may be because swiotlb_force is 
> > > > set 
> > > > to SWIOTLB_NO_FORCE, we will still enable the swiotlb when running on 
> > > > top 
> > > > of Xen.
> > > >
> > > > I am not entirely sure what would be the correct fix. Any opinions?
> > > 
> > > Can you try something like the patch below (not even compile tested, but
> > > the intent should be obvious?
> > > 
> > > 
> > > diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> > > index 16a2b2b1c54d..7671bc153fb1 100644
> > > --- a/arch/arm64/mm/init.c
> > > +++ b/arch/arm64/mm/init.c
> > > @@ -44,6 +44,8 @@
> > >  #include <asm/tlb.h>
> > >  #include <asm/alternative.h>
> > >  
> > > +#include <xen/arm/swiotlb-xen.h>
> > > +
> > >  /*
> > >   * We need to be able to catch inadvertent references to memstart_addr
> > >   * that occur (potentially in generic code) before arm64_memblock_init()
> > > @@ -482,7 +484,7 @@ void __init mem_init(void)
> > >   if (swiotlb_force == SWIOTLB_FORCE ||
> > >       max_pfn > PFN_DOWN(arm64_dma_phys_limit))
> > >           swiotlb_init(1);
> > > - else
> > > + else if (!IS_ENABLED(CONFIG_XEN) || !xen_swiotlb_detect())
> > >           swiotlb_force = SWIOTLB_NO_FORCE;
> > >  
> > >   set_max_mapnr(max_pfn - PHYS_PFN_OFFSET);
> > 
> > The "IS_ENABLED(CONFIG_XEN)" is not needed as the check is already part
> > of xen_swiotlb_detect().
> 
> As far as I can tell the x86 version of xen_swiotlb_detect has a
> !CONFIG_XEN stub.  The arm/arm64 version in uncoditionally declared, but
> the implementation only compiled when Xen support is enabled.

The implementation of xen_swiotlb_detect should work fine if
!CONFIG_XEN, but the issue is that it is implemented in
arch/arm/xen/mm.c, so it is not going to be available.

I think it would be good to turn it into a static inline so that we can
call it from arch/arm64/mm/init.c and other similar places with or
without CONFIG_XEN, see appended patch below. It compiles without
CONFIG_XEN.


> > But let me ask another question first. Do you think it makes sense to have:
> > 
> >     if (swiotlb_force == SWIOTLB_NO_FORCE)
> >             return 0;
> > 
> > at the beginning of swiotlb_late_init_with_tbl? I am asking because
> > swiotlb_late_init_with_tbl is meant for special late initializations,
> > right? It shouldn't really matter the presence or absence of
> > SWIOTLB_NO_FORCE in regards to swiotlb_late_init_with_tbl. Also the
> > commit message for "swiotlb: Make SWIOTLB_NO_FORCE perform no
> > allocation" says that "If a platform was somehow setting
> > swiotlb_no_force and a later call to swiotlb_init() was to be made we
> > would still be proceeding with allocating the default SWIOTLB size
> > (64MB)." Our case here is very similar, right? So the allocation should
> > proceed?
> 
> Well, right now SWIOTLB_NO_FORCE is checked in dma_direct_map_page.
> We need to clean all this up a bit, especially with the work to support
> multiple swiotlb buffers, but I think for now this is the best we can
> do.

OK


> > Which brings me to a separate unrelated issue, still affecting the path
> > xen_swiotlb_init -> swiotlb_late_init_with_tbl. If swiotlb_init(1) is
> > called by mem_init then swiotlb_late_init_with_tbl will fail due to the
> > check:
> > 
> >     /* protect against double initialization */
> >     if (WARN_ON_ONCE(io_tlb_default_mem))
> >         return -ENOMEM;
> > 
> > xen_swiotlb_init is meant to ask Xen to make a bunch of pages physically
> > contiguous. Then, it initializes the swiotlb buffer based on those
> > pages. So it is a problem that swiotlb_late_init_with_tbl refuses to
> > continue. However, in practice it is not a problem today because on ARM
> > we don't actually make any special requests to Xen to make the pages
> > physically contiguous (yet). See the empty implementation of
> > arch/arm/xen/mm.c:xen_create_contiguous_region. I don't know about x86.
> > 
> > So maybe we should instead do something like the appended?
> 
> So I'd like to change the core swiotlb initialization to just use
> a callback into the arch/xen code to make the pages contigous and
> kill all that code duplication.  Together with the multiple swiotlb
> buffer work I'd rather avoid churn that goes into a different direction
> if possible.

That's a much better plan. It is also not super urgent, so maybe for now
we could add an explicit check for io_tlb_default_mem != NULL at the
beginning of xen_swiotlb_init? So that at least we can fail explicitly
or ignore it explicitly rather than by accident.


---


diff --git a/arch/arm/xen/mm.c b/arch/arm/xen/mm.c
index f8f07469d259..223b1151fd7d 100644
--- a/arch/arm/xen/mm.c
+++ b/arch/arm/xen/mm.c
@@ -135,18 +135,6 @@ void xen_destroy_contiguous_region(phys_addr_t pstart, 
unsigned int order)
        return;
 }
 
-int xen_swiotlb_detect(void)
-{
-       if (!xen_domain())
-               return 0;
-       if (xen_feature(XENFEAT_direct_mapped))
-               return 1;
-       /* legacy case */
-       if (!xen_feature(XENFEAT_not_direct_mapped) && xen_initial_domain())
-               return 1;
-       return 0;
-}
-
 static int __init xen_mm_init(void)
 {
        struct gnttab_cache_flush cflush;
diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 16a2b2b1c54d..e55409caaee3 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -43,6 +43,7 @@
 #include <linux/sizes.h>
 #include <asm/tlb.h>
 #include <asm/alternative.h>
+#include <asm/xen/swiotlb-xen.h>
 
 /*
  * We need to be able to catch inadvertent references to memstart_addr
@@ -482,7 +483,7 @@ void __init mem_init(void)
        if (swiotlb_force == SWIOTLB_FORCE ||
            max_pfn > PFN_DOWN(arm64_dma_phys_limit))
                swiotlb_init(1);
-       else
+       else if (!xen_swiotlb_detect())
                swiotlb_force = SWIOTLB_NO_FORCE;
 
        set_max_mapnr(max_pfn - PHYS_PFN_OFFSET);
diff --git a/include/xen/arm/swiotlb-xen.h b/include/xen/arm/swiotlb-xen.h
index 2994fe6031a0..33336ab58afc 100644
--- a/include/xen/arm/swiotlb-xen.h
+++ b/include/xen/arm/swiotlb-xen.h
@@ -2,6 +2,19 @@
 #ifndef _ASM_ARM_SWIOTLB_XEN_H
 #define _ASM_ARM_SWIOTLB_XEN_H
 
-extern int xen_swiotlb_detect(void);
+#include <xen/features.h>
+#include <xen/xen.h>
+
+static inline int xen_swiotlb_detect(void)
+{
+       if (!xen_domain())
+               return 0;
+       if (xen_feature(XENFEAT_direct_mapped))
+               return 1;
+       /* legacy case */
+       if (!xen_feature(XENFEAT_not_direct_mapped) && xen_initial_domain())
+               return 1;
+       return 0;
+}
 
 #endif /* _ASM_ARM_SWIOTLB_XEN_H */