[Qemu-devel] Re: [PATCH] kvm/x86: enlarge number of possible CPUID leaves

[Andre Przywara <andre.przywara@amd.com>]

Avi Kivity wrote:
> On 12/01/2010 01:17 PM, Andre Przywara wrote:
>> Currently the number of CPUID leaves KVM handles is limited to 40.
>> My desktop machine (AthlonII) already has 35 and future CPUs will
>> expand this well beyond the limit. Extend the limit to 80 to make
>> room for future processors.
>>
>> Signed-off-by: Andre Przywara<andre.przywara@amd.com>
>> ---
>>   arch/x86/include/asm/kvm_host.h |    2 +-
>>   1 files changed, 1 insertions(+), 1 deletions(-)
>>
>> Hi,
>> I found that either KVM or QEMU (possibly both) are broken in respect
>> to handling more CPUID entries than the limit dictates. KVM will
>> return -E2BIG, which is the same error as if the user hasn't provided
>> enough space to hold all entries. Now QEMU will continue to enlarge
>> the allocated memory until it gets into an out-of-memory condition.
>> I have tried to fix this with teaching KVM how to deal with a capped
>> number of entries (there are some bugs in the current code), but this
>> will limit the number of CPUID entries KVM handles, which will surely
>> cut of the lastly appended PV leaves.
>> A proper fix would be to make this allocation dynamic. Is this a
>> feasible way or will this lead to issues or side-effects?
>>
> 
> Well, there has to be some limit, or userspace can allocate unbounded 
> kernel memory.
But this limit should not be a compile-time constant, but a runtime one. 
The needed size depends on the host CPU (plus the KVM PV leaves) and 
thus could be determined once for all VMs and vCPUs at module load-time. 
But then we cannot use the static array allocation we currently have in 
struct kvm_vcpu_arch:
     struct kvm_cpuid_entry2 cpuid_entries[KVM_MAX_CPUID_ENTRIES];
So we would use a kind-of dynamic allocation bounded by the host CPU's 
need. But for the code is does not make much difference to a "real" 
dynamic allocation.

Also we could implement kvm_dev_ioctl_get_supported_cpuid without the 
vmalloc, if we don't care about some dozens of copy_to_user() calls in 
this function. Then we would not need this limit in GET_SUPPORTED_CPUID 
at all, but it will strike us again at KVM_SET_CPUID[2], where we may 
not fulfill the promises we gave earlier.
Having said this, what about that:
kvm_dev_ioctl_get_supported_cpuid is invariant to the VM or vCPU (as it 
is used by a system ioctl), so it could be run once at initialization, 
which would limit the ioctl implementation to a plain bounded copy.
Would you want such a patch (removing the vmalloc and maybe even the limit)?

Regards,
Andre.

-- 
Andre Przywara
AMD-OSRC (Dresden)
Tel: x29712