Re: [Qemu-devel] [PATCH 1/2] KVM: Add new -cpu best
On 01/08/2012 05:52 PM, Alexander Graf wrote: During discussions on whether to make -cpu host the default in SLE, I found myself disagreeing to the thought, because it potentially opens a big can of worms for potential bugs. But if I already am so opposed to it for SLE, how can it possibly be reasonable to default to -cpu host in upstream QEMU? And what would a sane default look like? What are the arguments against -cpu host? Regards, Anthony Liguori
Re: [Qemu-devel] [PATCH 1/2] KVM: Add new -cpu best
On 16.01.2012, at 20:33, Anthony Liguori wrote: On 01/08/2012 05:52 PM, Alexander Graf wrote: During discussions on whether to make -cpu host the default in SLE, I found myself disagreeing to the thought, because it potentially opens a big can of worms for potential bugs. But if I already am so opposed to it for SLE, how can it possibly be reasonable to default to -cpu host in upstream QEMU? And what would a sane default look like? What are the arguments against -cpu host? It's hard to test. New CPUs have new features and we're having a hard time to catch up. With -cpu best we only select from a pool of known-good CPU types. If you want to check that everything works, go to a box that has the maximum available features, go through all -cpu options that users could run into and you're good. With -cpu host you can't really test (unless you own all possible CPUs there are). We expose CPUID information that doesn't exist that way in the real world. A small example from today's code. There are a bunch of CPUID leafs. On Nehalem, one of them is a list of possible C-States to go into. With -cpu host we sync feature bits, CPU name, CPU family and some other bits of information, but not the C-State information. So we end up with a CPU inside the guest that looks and feels like a Nehalem CPU, but doesn't expose any C-State information. Linux now boots, goes in, checks that it's running on Nehalem, sets the powersave mechanism to the respective model and fills an internal callback table with the C-State information with a loop that ends without any action, since we expose 0 C-State bits. When the guest now calls the idle callback, it dereferences that table, which contains a NULL pointer, oops. That is just one example from current Linux. Another one would be my development AMD box that when it came out wasn't around in the market yet, so guests would just refuse to boot at all. Since they'd just say the CPUID is unknown. Overall, I used to be a big fan of -cpu host, but it's a maintainability nightmare. It can be great for testing stuff, so we should definitely keep it around. But after thinking about it again, I don't think it should be the default. The default should be something safe. Alex
Re: [Qemu-devel] [PATCH 1/2] KVM: Add new -cpu best
On 8 January 2012 23:52, Alexander Graf ag...@suse.de wrote: During discussions on whether to make -cpu host the default in SLE, I found myself disagreeing to the thought, because it potentially opens a big can of worms for potential bugs. But if I already am so opposed to it for SLE, how can it possibly be reasonable to default to -cpu host in upstream QEMU? And what would a sane default look like? So I had this idea of looping through all available CPU definitions. We can pretty well tell if our host is able to execute any of them by checking the respective flags and seeing if our host has all features the CPU definition requires. With that, we can create a -cpu type that would fall back to the best known CPU definition that our host can fulfill. On my Phenom II system for example, that would be -cpu phenom. ...shouldn't this be supported on at least all hosts with KVM support, not just x86? Also I don't see any documentation updates in this patchset :-) -- PMM
Re: [Qemu-devel] [PATCH 1/2] KVM: Add new -cpu best
On 09.01.2012, at 01:02, Peter Maydell wrote: On 8 January 2012 23:52, Alexander Graf ag...@suse.de wrote: During discussions on whether to make -cpu host the default in SLE, I found myself disagreeing to the thought, because it potentially opens a big can of worms for potential bugs. But if I already am so opposed to it for SLE, how can it possibly be reasonable to default to -cpu host in upstream QEMU? And what would a sane default look like? So I had this idea of looping through all available CPU definitions. We can pretty well tell if our host is able to execute any of them by checking the respective flags and seeing if our host has all features the CPU definition requires. With that, we can create a -cpu type that would fall back to the best known CPU definition that our host can fulfill. On my Phenom II system for example, that would be -cpu phenom. ...shouldn't this be supported on at least all hosts with KVM support, not just x86? I don't think it makes sense on any other platform. For PPC -cpu host is good enough, since it's basically doing the same as -cpu best. We only have a single 32 bit number as identifier for -cpu host there. Also I don't see any documentation updates in this patchset :-) Do we have documentation for -cpu host? Alex
