Re: [PATCH V2 4/4] arm/xen: Read extended regions from DT and init Xen resource

[Stefano Stabellini <sstabellini@kernel.org>]

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On Fri, 19 Nov 2021, Oleksandr wrote:
> On 19.11.21 03:19, Stefano Stabellini wrote:
> > On Wed, 10 Nov 2021, Oleksandr wrote:
> > > On 28.10.21 04:40, 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@epam.com>
> > > > > 
> > > > > This patch implements arch_xen_unpopulated_init() on Arm where
> > > > > the extended regions (if any) are gathered from DT and inserted
> > > > > into passed Xen resource to be used as unused address space
> > > > > for Xen scratch pages by unpopulated-alloc code.
> > > > > 
> > > > > The extended region (safe range) is a region of guest physical
> > > > > address space which is unused and could be safely used to create
> > > > > grant/foreign mappings instead of wasting real RAM pages from
> > > > > the domain memory for establishing these mappings.
> > > > > 
> > > > > The extended regions are chosen by the hypervisor at the domain
> > > > > creation time and advertised to it via "reg" property under
> > > > > hypervisor node in the guest device-tree. As region 0 is reserved
> > > > > for grant table space (always present), the indexes for extended
> > > > > regions are 1...N.
> > > > > 
> > > > > If arch_xen_unpopulated_init() fails for some reason the default
> > > > > behaviour will be restored (allocate xenballooned pages).
> > > > > 
> > > > > This patch also removes XEN_UNPOPULATED_ALLOC dependency on x86.
> > > > > 
> > > > > Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> > > > > ---
> > > > > Changes RFC -> V2:
> > > > >      - new patch, instead of
> > > > >       "[RFC PATCH 2/2] xen/unpopulated-alloc: Query hypervisor to
> > > > > provide
> > > > > unallocated space"
> > > > > ---
> > > > >    arch/arm/xen/enlighten.c | 112
> > > > > +++++++++++++++++++++++++++++++++++++++++++++++
> > > > >    drivers/xen/Kconfig      |   2 +-
> > > > >    2 files changed, 113 insertions(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
> > > > > index dea46ec..1a1e0d3 100644
> > > > > --- a/arch/arm/xen/enlighten.c
> > > > > +++ b/arch/arm/xen/enlighten.c
> > > > > @@ -62,6 +62,7 @@ static __read_mostly unsigned int xen_events_irq;
> > > > >    static phys_addr_t xen_grant_frames;
> > > > >      #define GRANT_TABLE_INDEX   0
> > > > > +#define EXT_REGION_INDEX    1
> > > > >      uint32_t xen_start_flags;
> > > > >    EXPORT_SYMBOL(xen_start_flags);
> > > > > @@ -303,6 +304,117 @@ static void __init xen_acpi_guest_init(void)
> > > > >    #endif
> > > > >    }
> > > > >    +#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
> > > > > +int arch_xen_unpopulated_init(struct resource *res)
> > > > > +{
> > > > > +	struct device_node *np;
> > > > > +	struct resource *regs, *tmp_res;
> > > > > +	uint64_t min_gpaddr = -1, max_gpaddr = 0;
> > > > > +	unsigned int i, nr_reg = 0;
> > > > > +	struct range mhp_range;
> > > > > +	int rc;
> > > > > +
> > > > > +	if (!xen_domain())
> > > > > +		return -ENODEV;
> > > > > +
> > > > > +	np = of_find_compatible_node(NULL, NULL, "xen,xen");
> > > > > +	if (WARN_ON(!np))
> > > > > +		return -ENODEV;
> > > > > +
> > > > > +	/* Skip region 0 which is reserved for grant table space */
> > > > > +	while (of_get_address(np, nr_reg + EXT_REGION_INDEX, NULL,
> > > > > NULL))
> > > > > +		nr_reg++;
> > > > > +	if (!nr_reg) {
> > > > > +		pr_err("No extended regions are found\n");
> > > > > +		return -EINVAL;
> > > > > +	}
> > > > > +
> > > > > +	regs = kcalloc(nr_reg, sizeof(*regs), GFP_KERNEL);
> > > > > +	if (!regs)
> > > > > +		return -ENOMEM;
> > > > > +
> > > > > +	/*
> > > > > +	 * Create resource from extended regions provided by the
> > > > > hypervisor to
> > > > > be
> > > > > +	 * used as unused address space for Xen scratch pages.
> > > > > +	 */
> > > > > +	for (i = 0; i < nr_reg; i++) {
> > > > > +		rc = of_address_to_resource(np, i + EXT_REGION_INDEX,
> > > > > &regs[i]);
> > > > > +		if (rc)
> > > > > +			goto err;
> > > > > +
> > > > > +		if (max_gpaddr < regs[i].end)
> > > > > +			max_gpaddr = regs[i].end;
> > > > > +		if (min_gpaddr > regs[i].start)
> > > > > +			min_gpaddr = regs[i].start;
> > > > > +	}
> > > > > +
> > > > > +	/* Check whether the resource range is within the hotpluggable
> > > > > range
> > > > > */
> > > > > +	mhp_range = mhp_get_pluggable_range(true);
> > > > > +	if (min_gpaddr < mhp_range.start)
> > > > > +		min_gpaddr = mhp_range.start;
> > > > > +	if (max_gpaddr > mhp_range.end)
> > > > > +		max_gpaddr = mhp_range.end;
> > > > > +
> > > > > +	res->start = min_gpaddr;
> > > > > +	res->end = max_gpaddr;
> > > > > +
> > > > > +	/*
> > > > > +	 * Mark holes between extended regions as unavailable. The
> > > > > rest of
> > > > > that
> > > > > +	 * address space will be available for the allocation.
> > > > > +	 */
> > > > > +	for (i = 1; i < nr_reg; i++) {
> > > > > +		resource_size_t start, end;
> > > > > +
> > > > > +		start = regs[i - 1].end + 1;
> > > > > +		end = regs[i].start - 1;
> > > > > +
> > > > > +		if (start > (end + 1)) {
> > > > Should this be:
> > > > 
> > > > if (start >= end)
> > > > 
> > > > ?
> > > Yes, we can do this here (since the checks are equivalent) but ...
> > > 
> > > > > +			rc = -EINVAL;
> > > > > +			goto err;
> > > > > +		}
> > > > > +
> > > > > +		/* There is no hole between regions */
> > > > > +		if (start == (end + 1))
> > > > Also here, shouldn't it be:
> > > > 
> > > > if (start == end)
> > > > 
> > > > ?
> > >     ... not here.
> > > 
> > > As
> > > 
> > > "(start == (end + 1))" is equal to "(regs[i - 1].end + 1 ==
> > > regs[i].start)"
> > > 
> > > but
> > > 
> > > "(start == end)" is equal to "(regs[i - 1].end + 1 == regs[i].start - 1)"
> >   OK. But the check:
> > 
> >    if (start >= end)
> > 
> > Actually covers both cases so that's the only check we need?
> 
> Sorry, I don't entirely understand the question.
> Is the question to use only a single check in that loop?
> 
> Paste the updated code which I have locally for the convenience.
> 
>  [snip]
> 
>     /*
>      * Mark holes between extended regions as unavailable. The rest of that
>      * address space will be available for the allocation.
>      */
>     for (i = 1; i < nr_reg; i++) {
>         resource_size_t start, end;
> 
>         start = regs[i - 1].end + 1;
>         end = regs[i].start - 1;
> 
>         if (start > (end + 1)) {
>             rc = -EINVAL;
>             goto err;
>         }
> 
>         /* There is no hole between regions */
>         if (start == (end + 1))
>             continue;
> 
>         tmp_res = kzalloc(sizeof(*tmp_res), GFP_KERNEL);
>         if (!tmp_res) {
>             rc = -ENOMEM;
>             goto err;
>         }
> 
>         tmp_res->name = "Unavailable space";
>         tmp_res->start = start;
>         tmp_res->end = end;
> 
>         rc = insert_resource(&xen_resource, tmp_res);
>         if (rc) {
>             pr_err("Cannot insert resource %pR (%d)\n", tmp_res, rc);
>             kfree(tmp_res);
>             goto err;
>         }
>     }
> 
> [snip]
> 
> 
> 1. The first check is to detect an overlap (which is a wrong configuration,
> correct?) and bail out if true (for example, regX: 0x81000000...0x82FFFFFF and
> regY: 0x82000000...0x83FFFFFF).
> 2. The second check is just to skip current iteration as there is no
> space/hole between regions (for example, regX: 0x81000000...0x82FFFFFF and
> regY: 0x83000000...0x83FFFFFF).
> Therefore I think they should be distinguished.
> 
> Yes, both check could be transformed to a single one, but this way the
> overlaps will be ignored:
> if (start >= (end + 1))
>     continue;
> 
> Or I really missed something?

You are right it is better to distinguish the two cases. I suggest the
code below because I think it is a clearer, even if it might be slightly
less efficient. I don't feel too strongly about it though.

		resource_size_t start, end;

		/* There is no hole between regions */
		if ( regs[i - 1].end + 1 == regs[i].start )
			continue;

		if ( regs[i - 1].end + 1 > regs[i].start) {
			rc = -EINVAL;
			goto err;
		}

		start = regs[i - 1].end + 1;
		end = regs[i].start - 1;