linux-kernel - Re: [PATCH V3 2/6] arm/xen: Switch to use gnttab_setup_auto_xlat

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Message-ID: <f69cb505-d14f-c37c-cccb-49d79a66af54@gmail.com>
Date:   Thu, 25 Nov 2021 13:41:27 +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 V3 2/6] arm/xen: Switch to use
 gnttab_setup_auto_xlat_frames() for DT


On 25.11.21 01:35, Stefano Stabellini wrote:

Hi Stefano


> On Wed, 24 Nov 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(). Also in order to make a code more
>> resilient use a fallback to xen_xlate_map_ballooned_pages() if grant
>> table region wasn't found.
>>
>> 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
>>
>> Changes V2 -> V3:
>>     - add __read_mostly specifier to xen_grant_frames
>>     - retain a fallback to xen_xlate_map_ballooned_pages() if
>>       xen_grant_frames is invalid
>>     - process xen_events_irq before xen_grant_frames in
>>       xen_dt_guest_init()
>>     - update patch description
>> ---
>>   arch/arm/xen/enlighten.c | 26 ++++++++++++++++++++------
>>   1 file changed, 20 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
>> index 7619fbf..3fb3384 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 __read_mostly phys_addr_t xen_grant_frames;
>> +
>> +#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) {
>> @@ -311,13 +315,19 @@ static void __init xen_dt_guest_init(void)
>>   	}
>>   
>>   	xen_events_irq = irq_of_parse_and_map(xen_node, 0);
>> +
>> +	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;
>>   }
>>   
>>   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;
>> @@ -370,12 +380,16 @@ 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 || !xen_grant_frames) {
> I realize now that we only need:
>
>      if (!xen_grant_frames) {

Indeed, the first part of the check looks superfluous, will do.


>
> with that:
>
> Reviewed-by: Stefano Stabellini <sstabellini@...nel.org>

Thanks!


>
>
>
>> +		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