From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40808) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Zr0F9-0007Sr-CB for qemu-devel@nongnu.org; Tue, 27 Oct 2015 05:08:36 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Zr0F4-0000MJ-CD for qemu-devel@nongnu.org; Tue, 27 Oct 2015 05:08:35 -0400 Received: from mx1.redhat.com ([209.132.183.28]:40797) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Zr0F4-0000MF-71 for qemu-devel@nongnu.org; Tue, 27 Oct 2015 05:08:30 -0400 Received: from int-mx14.intmail.prod.int.phx2.redhat.com (int-mx14.intmail.prod.int.phx2.redhat.com [10.5.11.27]) by mx1.redhat.com (Postfix) with ESMTPS id AAF014C0A1 for ; Tue, 27 Oct 2015 09:08:29 +0000 (UTC) Date: Tue, 27 Oct 2015 11:08:26 +0200 From: "Michael S. Tsirkin" Message-ID: <20151027110716-mutt-send-email-mst@redhat.com> References: <1445848925-84796-1-git-send-email-imammedo@redhat.com> <20151026183318.GF4180@thinpad.lan.raisama.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151026183318.GF4180@thinpad.lan.raisama.net> Subject: Re: [Qemu-devel] [PATCH] pc: memhp: enforce minimal 128Mb alignment for pc-dimm List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Eduardo Habkost Cc: Igor Mammedov , qemu-devel@nongnu.org, pbonzini@redhat.com On Mon, Oct 26, 2015 at 04:33:18PM -0200, Eduardo Habkost wrote: > On Mon, Oct 26, 2015 at 09:42:05AM +0100, Igor Mammedov wrote: > > commit aa8580cd "pc: memhp: force gaps between DIMM's GPA" > > regressed memory hot-unplug for linux guests triggering > > following BUGON > > ===== > > kernel BUG at mm/memory_hotplug.c:703! > > ... > > [] acpi_memory_device_remove+0x79/0xa5 > > [] acpi_bus_trim+0x5a/0x8d > > [] acpi_device_hotplug+0x1b7/0x418 > > === > > BUG_ON(phys_start_pfn & ~PAGE_SECTION_MASK); > > === > > > > reson for it is that x86-64 linux guest supports memory > > hotplug in chunks of 128Mb and memory section also should > > be 128Mb aligned. > > However gaps forced between 128Mb DIMMs with backend's > > natural alignment of 2Mb make the 2nd and following > > DIMMs not being aligned on 128Mb boundary as it was > > originally. To fix regression enforce minimal 128Mb > > alignment like it was done for PPC. > > > > Signed-off-by: Igor Mammedov > > --- > > hw/i386/pc.c | 5 +++++ > > 1 file changed, 5 insertions(+) > > > > diff --git a/hw/i386/pc.c b/hw/i386/pc.c > > index 3d958ba..cd68169 100644 > > --- a/hw/i386/pc.c > > +++ b/hw/i386/pc.c > > @@ -1610,6 +1610,8 @@ void ioapic_init_gsi(GSIState *gsi_state, const char *parent_name) > > } > > } > > > > +#define MIN_DIMM_ALIGNMENT (1ULL << 27) /* 128Mb */ > > If you send a new version, could you include the explanation for the > 128MB value as a comment above the macro definition? The issue is that there's no good explanation yet. It's just something that seems to work for current linux. Why does linux do it, and what basis does it have in hardware, IIUC we don't know. > -- > Eduardo