Re: [PATCH V3 2/6] arm/xen: Switch to use gnttab_setup_auto_xlat_frames() for DT

[Oleksandr]

On 25.11.21 01:35, Stefano Stabellini wrote:

Hi Stefano


> On Wed, 24 Nov 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(). Also in order to make a code more
> > resilient use a fallback to xen_xlate_map_ballooned_pages() if grant
> > table region wasn't found.
> >
> > 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
> >
> > Changes V2 -> V3:
> >     - add __read_mostly specifier to xen_grant_frames
> >     - retain a fallback to xen_xlate_map_ballooned_pages() if
> >       xen_grant_frames is invalid
> >     - process xen_events_irq before xen_grant_frames in
> >       xen_dt_guest_init()
> >     - update patch description
> > ---
> >   arch/arm/xen/enlighten.c | 26 ++++++++++++++++++++------
> >   1 file changed, 20 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
> > index 7619fbf..3fb3384 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 __read_mostly phys_addr_t xen_grant_frames;
> > +
> > +#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) {
> > @@ -311,13 +315,19 @@ static void __init xen_dt_guest_init(void)
> >   	}
> >   
> >   	xen_events_irq = irq_of_parse_and_map(xen_node, 0);
> > +
> > +	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;
> >   }
> >   
> >   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;
> > @@ -370,12 +380,16 @@ 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 || !xen_grant_frames) {
> I realize now that we only need:
>
>      if (!xen_grant_frames) {

Indeed, the first part of the check looks superfluous, will do.


> with that:
>
> Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>

Thanks!


> > +		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();

--
Regards,

Oleksandr Tyshchenko