From mboxrd@z Thu Jan 1 00:00:00 1970 From: Juergen Gross Subject: Re: [PATCH] expand x86 arch_shared_info to support >3 level p2m tree Date: Mon, 08 Sep 2014 18:11:25 +0200 Message-ID: <540DD52D.3030307@suse.com> References: <1410184138-11277-1-git-send-email-jgross@suse.com> <1410184138-11277-2-git-send-email-jgross@suse.com> <540DB65F.30601@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <540DB65F.30601@citrix.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Andrew Cooper , ian.campbell@citrix.com, ian.jackson@eu.citrix.com, jbeulich@suse.com, keir@xen.org, tim@xen.org, xen-devel@lists.xen.org List-Id: xen-devel@lists.xenproject.org On 09/08/2014 03:59 PM, Andrew Cooper wrote: > On 08/09/14 14:48, Juergen Gross wrote: >> The x86 struct arch_shared_info field pfn_to_mfn_frame_list_list >> currently contains the mfn of the top level page frame of the 3 level >> p2m tree, which is used by the Xen tools during saving and restoring >> (and live migration) of pv domains. With three levels of the p2m tree >> it is possible to support up to 512 GB of RAM for a pv domain. > > Specifically only 64bit PV domains have the 512GB limit. > > 32bit PV domains have a far larger supported RAM as they can fit twice > as many mfns in each p2m page. > >> To be >> able to support more RAM an additional level is to be added. >> >> This patch expands struct arch_shared_info with a new p2m tree root >> and the number of levels of the p2m tree. The new information is >> indicated by the domain to be valid by storing ~0UL into >> pfn_to_mfn_frame_list_list (this should be done only if more than >> three levels are needed, of course). >> >> Signed-off-by: Juergen Gross >> --- >> xen/include/public/arch-x86/xen.h | 8 +++++++- >> 1 file changed, 7 insertions(+), 1 deletion(-) >> >> diff --git a/xen/include/public/arch-x86/xen.h b/xen/include/public/arch-x86/xen.h >> index f35804b..b7fa2b6 100644 >> --- a/xen/include/public/arch-x86/xen.h >> +++ b/xen/include/public/arch-x86/xen.h >> @@ -224,7 +224,13 @@ struct arch_shared_info { >> /* Frame containing list of mfns containing list of mfns containing p2m. */ >> xen_pfn_t pfn_to_mfn_frame_list_list; >> unsigned long nmi_reason; >> - uint64_t pad[32]; >> + /* >> + * Following two fields are valid if pfn_to_mfn_frame_list_list contains >> + * ~0UL. >> + */ >> + unsigned long p2m_levels; /* number of levels of p2m tree */ >> + xen_pfn_t p2m_root; /* p2m tree top level mfn */ >> + uint64_t pad[30]; > > This padding is now wrong, as unsigned long is only 4 bytes in 32bit, > and it is adjacent to another unsigned long. The padding in this case is a nightmare. For 32 bits arch_shared_info_t is 64 bit aligned due to uint64_t pad[]. This enforces a 4 byte hole before the structure (there are 3 4-byte fields before it in shared_info_t). And before pad[] there is another 4-byte hole. And don't forget: on 32 bits xen_pfn_t is 4 bytes, too. I could either align each new variable explicitly to 8 bytes or I could use a union. As arch_shared_info_t is at the end of shared_info_t I could just ignore the alignment/padding ... What is your preference? Juergen