Re: [PATCH V2 2/4] arm/xen: Switch to use gnttab_setup_auto_xlat_frames() for DT

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 11 Nov 2021, Oleksandr wrote:
> On 28.10.21 04:28, Stefano Stabellini wrote:
> 
> Hi Stefano
> 
> I am sorry for the late response.
> 
> > On Tue, 26 Oct 2021, Oleksandr Tyshchenko wrote:
> > > From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> > > 
> > > Read the start address of the grant table space from DT
> > > (region 0).
> > > 
> > > This patch mostly restores behaviour before commit 3cf4095d7446
> > > ("arm/xen: Use xen_xlate_map_ballooned_pages to setup grant table")
> > > but trying not to break the ACPI support added after that commit.
> > > So the patch touches DT part only and leaves the ACPI part with
> > > xen_xlate_map_ballooned_pages().
> > > 
> > > This is a preparation for using Xen extended region feature
> > > where unused regions of guest physical address space (provided
> > > by the hypervisor) will be used to create grant/foreign/whatever
> > > mappings instead of wasting real RAM pages from the domain memory
> > > for establishing these mappings.
> > > 
> > > The immediate benefit of this change:
> > > - Avoid superpage shattering in Xen P2M when establishing
> > >    stage-2 mapping (GFN <-> MFN) for the grant table space
> > > - Avoid wasting real RAM pages (reducing the amount of memory
> > >    usuable) for mapping grant table space
> > > - The grant table space is always mapped at the exact
> > >    same place (region 0 is reserved for the grant table)
> > > 
> > > Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> > > ---
> > > Changes RFC -> V2:
> > >     - new patch
> > > ---
> > >   arch/arm/xen/enlighten.c | 32 +++++++++++++++++++++++++-------
> > >   1 file changed, 25 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
> > > index 7f1c106b..dea46ec 100644
> > > --- a/arch/arm/xen/enlighten.c
> > > +++ b/arch/arm/xen/enlighten.c
> > > @@ -59,6 +59,9 @@ unsigned long xen_released_pages;
> > >   struct xen_memory_region xen_extra_mem[XEN_EXTRA_MEM_MAX_REGIONS]
> > > __initdata;
> > >     static __read_mostly unsigned int xen_events_irq;
> > > +static phys_addr_t xen_grant_frames;
> > __read_mostly
> 
> ok
> 
> 
> > 
> > 
> > > +#define GRANT_TABLE_INDEX   0
> > >     uint32_t xen_start_flags;
> > >   EXPORT_SYMBOL(xen_start_flags);
> > > @@ -303,6 +306,7 @@ static void __init xen_acpi_guest_init(void)
> > >   static void __init xen_dt_guest_init(void)
> > >   {
> > >           struct device_node *xen_node;
> > > + struct resource res;
> > >           xen_node = of_find_compatible_node(NULL, NULL, "xen,xen");
> > >           if (!xen_node) {
> > > @@ -310,6 +314,12 @@ static void __init xen_dt_guest_init(void)
> > >                   return;
> > >           }
> > >   +       if (of_address_to_resource(xen_node, GRANT_TABLE_INDEX, &res)) {
> > > +         pr_err("Xen grant table region is not found\n");
> > > +         return;
> > > + }
> > > + xen_grant_frames = res.start;
> > > +
> > >           xen_events_irq = irq_of_parse_and_map(xen_node, 0);
> > >   }
> > >   @@ -317,16 +327,20 @@ static int __init xen_guest_init(void)
> > >   {
> > >           struct xen_add_to_physmap xatp;
> > >           struct shared_info *shared_info_page = NULL;
> > > - int cpu;
> > > + int rc, cpu;
> > >           if (!xen_domain())
> > >                   return 0;
> > >           if (!acpi_disabled)
> > >                   xen_acpi_guest_init();
> > > - else
> > > + else {
> > >                   xen_dt_guest_init();
> > >   +               if (!xen_grant_frames)
> > > +                 return -ENODEV;
> > maybe we can avoid this, see below
> > 
> > 
> > > + }
> > > +
> > >           if (!xen_events_irq) {
> > >                   pr_err("Xen event channel interrupt not found\n");
> > >                   return -ENODEV;
> > > @@ -370,12 +384,16 @@ static int __init xen_guest_init(void)
> > >           for_each_possible_cpu(cpu)
> > >                   per_cpu(xen_vcpu_id, cpu) = cpu;
> > >   -       xen_auto_xlat_grant_frames.count = gnttab_max_grant_frames();
> > > - if (xen_xlate_map_ballooned_pages(&xen_auto_xlat_grant_frames.pfn,
> > > -                                   &xen_auto_xlat_grant_frames.vaddr,
> > > -                                   xen_auto_xlat_grant_frames.count)) {
> > > + if (!acpi_disabled) {
> > To make the code more resilient couldn't we do:
> > 
> > if (!acpi_disabled || !xen_grant_frames) {
> I think, we can.
> 
> On the one hand, indeed the code more resilient and less change.
> From the other hand if grant table region is not found then something weird
> happened as region 0 is always present in reg property if hypervisor node is
> exposed to the guest.
> The behavior before commit 3cf4095d7446 ("arm/xen: Use
> xen_xlate_map_ballooned_pages to setup grant table") was exactly the same in
> the context of the failure if region wasn't found.
> 
> ...
> 
> Well, if we want to make code more resilient, I will update. But, looks like
> we also need to switch actions in xen_dt_guest_init() in order to process
> xen_events_irq before xen_grant_frames, otherwise we may return after failing
> with region and end up not initializing xen_events_irq so xen_guest_init()
> will fail earlier than reaches that check.
> What do you think?
 
Yes, you are right. I was re-reading the patch to refresh my memory and
I noticed immediately that xen_dt_guest_init also need to be changed so
that xen_events_irq is set before xen_grant_frames.
 
I think it is a minor change that doesn't add complexity but make the
code more robust so I think it is a good idea

 
> > > +         xen_auto_xlat_grant_frames.count = gnttab_max_grant_frames();
> > > +         rc =
> > > xen_xlate_map_ballooned_pages(&xen_auto_xlat_grant_frames.pfn,
> > > +
> > > &xen_auto_xlat_grant_frames.vaddr,
> > > +
> > > xen_auto_xlat_grant_frames.count);
> > > + } else
> > > +         rc = gnttab_setup_auto_xlat_frames(xen_grant_frames);
> > > + if (rc) {
> > >                   free_percpu(xen_vcpu_info);
> > > -         return -ENOMEM;
> > > +         return rc;
> > >           }
> > >           gnttab_init();