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

[Oleksandr <olekstysh@gmail.com>]

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@epam.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@epam.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