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Message-ID: <1c32b559-16f0-c993-eca3-86cf70f97132@gmail.com>
Date: Fri, 19 Nov 2021 20:25:16 +0200
From: Oleksandr <olekstysh@...il.com>
To: Stefano Stabellini <sstabellini@...nel.org>
Cc: 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>,
Julien Grall <julien@....org>
Subject: Re: [PATCH V2 2/4] arm/xen: Switch to use
gnttab_setup_auto_xlat_frames() for DT
On 19.11.21 02:32, Stefano Stabellini wrote:
Hi Stefano
> 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@...m.com>
>>>>
>>>> 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@...m.com>
>>>> ---
>>>> 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
Great, thank you. Will do in next version.
>
>
>>>> + 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
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