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Message-ID: <alpine.DEB.2.22.394.2111191820130.1412361@ubuntu-linux-20-04-desktop>
Date: Fri, 19 Nov 2021 18:36:35 -0800 (PST)
From: Stefano Stabellini <sstabellini@...nel.org>
To: Oleksandr <olekstysh@...il.com>
cc: Stefano Stabellini <sstabellini@...nel.org>,
xen-devel@...ts.xenproject.org,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
Russell King <linux@...linux.org.uk>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>,
Juergen Gross <jgross@...e.com>, Julien Grall <julien@....org>
Subject: Re: [PATCH V2 4/4] arm/xen: Read extended regions from DT and init
Xen resource
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@...m.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@...m.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,
> > > > > ®s[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;
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