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

[Juergen Gross <jgross@suse.com>]

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@suse.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.


Juergen