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

[Oleksandr]

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?


> > +		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