Am 30.05.2013 10:18, schrieb Alexey Kardashevskiy: > On 05/30/2013 05:49 PM, Paolo Bonzini wrote: >> Il 30/05/2013 09:44, Alexey Kardashevskiy ha scritto: >>> Hi! >>> >>> I found the migration broken on pseries platform, specifically, this patch >>> broke it: >>> >>> f1c72795af573b24a7da5eb52375c9aba8a37972 >>> migration: do not sent zero pages in bulk stage >>> >>> The idea is not to send zero pages to the destination guest which is >>> expected to have 100% empty RAM. >>> >>> However on pseries plaftorm the guest always has some stuff in the RAM as a >>> part of initialization (device tree, system firmware and rtas (?)) so it is >>> not completely empty. As the source guest cannot detect this, it skips some >>> pages during migration and we get a broken destination guest. Bug. >>> >>> While the idea is ok in general, I do not see any easy way to fix it as >>> neither QEMUMachine::init nor QEMUMachine::reset callbacks has information >>> about whether we are about to receive a migration or not (-incoming >>> parameter) and we cannot move device-tree and system firmware >>> initialization anywhere else. >>> >>> ram_bulk_stage is static and cannot be disabled from the platform >>> initialization code. >>> >>> So what would the community suggest? >> Revert the patch. :) > I'll wait for 24 hours (forgot to cc: the author) and then post a revert > patch :) > > > does this problem only occur on pseries emulation?
not sending zero pages is not only a performance benefit it also makes overcomitted memory usable. the madv_dontneed seems to kick in asynchronously and memory is not available immediately. what I do not understand if the a memory region is not empty at destination due to device tree, firmware etc. it shouldn't be empty at the source as well so in theory this should not be a problem. Peter