linux-kernel - Re: [PATCH] xen: allocate page for shared info page from low memory

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Date:   Mon, 24 Jul 2017 23:58:51 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH] xen: allocate page for shared info page from low memory



On 07/23/2017 04:25 PM, Boris Ostrovsky wrote:
> 
> 
> On 06/14/2017 01:11 PM, Juergen Gross wrote:
>> On 14/06/17 18:58, Boris Ostrovsky wrote:
>>> On 06/12/2017 07:53 AM, Juergen Gross wrote:
>>>> In a HVM guest the kernel allocates the page for mapping the shared
>>>> info structure via extend_brk() today. This will lead to a drop of
>>>> performance as the underlying EPT entry will have to be split up into
>>>> 4kB entries as the single shared info page is located in hypervisor
>>>> memory.
>>>>
>>>> The issue has been detected by using the libmicro munmap test:
>>>> unmapping 8kB of memory was faster by nearly a factor of two when no
>>>> pv interfaces were active in the HVM guest.
>>>>
>>>> So instead of taking a page from memory which might be mapped via
>>>> large EPT entries use a page which is already mapped via a 4kB EPT
>>>> entry: we can take a page from the first 1MB of memory as the video
>>>> memory at 640kB disallows using larger EPT entries.
>>>>
>>>> Signed-off-by: Juergen Gross <jgross@...e.com>
>>>> ---
>>>>   arch/x86/xen/enlighten_hvm.c | 31 ++++++++++++++++++++++++-------
>>>>   arch/x86/xen/enlighten_pv.c  |  2 --
>>>>   2 files changed, 24 insertions(+), 9 deletions(-)
>>>>
>>>> diff --git a/arch/x86/xen/enlighten_hvm.c 
>>>> b/arch/x86/xen/enlighten_hvm.c
>>>> index a6d014f47e52..c19477b6e43a 100644
>>>> --- a/arch/x86/xen/enlighten_hvm.c
>>>> +++ b/arch/x86/xen/enlighten_hvm.c
>>>> @@ -1,5 +1,6 @@
>>>>   #include <linux/cpu.h>
>>>>   #include <linux/kexec.h>
>>>> +#include <linux/memblock.h>
>>>>   #include <xen/features.h>
>>>>   #include <xen/events.h>
>>>> @@ -10,9 +11,11 @@
>>>>   #include <asm/reboot.h>
>>>>   #include <asm/setup.h>
>>>>   #include <asm/hypervisor.h>
>>>> +#include <asm/e820/api.h>
>>>>   #include <asm/xen/cpuid.h>
>>>>   #include <asm/xen/hypervisor.h>
>>>> +#include <asm/xen/page.h>
>>>>   #include "xen-ops.h"
>>>>   #include "mmu.h"
>>>> @@ -22,20 +25,34 @@ void __ref xen_hvm_init_shared_info(void)
>>>>   {
>>>>       int cpu;
>>>>       struct xen_add_to_physmap xatp;
>>>> -    static struct shared_info *shared_info_page;
>>>> +    u64 pa;
>>>> +
>>>> +    if (HYPERVISOR_shared_info == &xen_dummy_shared_info) {
>>>> +        /*
>>>> +         * Search for a free page starting at 4kB physical address.
>>>> +         * Low memory is preferred to avoid an EPT large page split up
>>>> +         * by the mapping.
>>>> +         * Starting below X86_RESERVE_LOW (usually 64kB) is fine as
>>>> +         * the BIOS used for HVM guests is well behaved and won't
>>>> +         * clobber memory other than the first 4kB.
>>>> +         */
>>>> +        for (pa = PAGE_SIZE;
>>>> +             !e820__mapped_all(pa, pa + PAGE_SIZE, E820_TYPE_RAM) ||
>>>> +             memblock_is_reserved(pa);
>>>> +             pa += PAGE_SIZE)
>>>> +            ;
>>>
>>> Is it possible to never find a page here?
>>
>> Only if there is no memory available at all. :-)
>>
>> TBH: I expect this to _always_ succeed at the first loop iteration.
> 
> This patch seems to break (64-bit only) guests on dumpdata here. No 
> problems on other machines.
> 
> So far all I know is that we did get the first page (0x1000) but not 
> much more. I will poke at this more on Monday.


So the problem is due to KASLR --- we can't use __va() before 
kernel_randomize_memory() is called since it will change __PAGE_OFFSET. 
(Setting CONFIG_RANDOMIZE_BASE will cause failure.)


-boris