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

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Message-ID: <75df36ae-bcc2-62e9-b585-cc2b74a682de@oracle.com>
Date:   Sun, 23 Jul 2017 16:25:27 -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 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.

-boris