Re: [Qemu-devel] [PATCH 7/7] KVM regsync: Fix do_kvm_cpu_synchronize_state data integrity issue

["Jason J. Herne" <jjherne@linux.vnet.ibm.com>]

On 01/04/2013 11:27 AM, Alexander Graf wrote:
>
> On 04.01.2013, at 16:25, Jason J. Herne wrote:
>
>> If I've followed the conversation correctly this is what needs to be done:
>>
>> 1. Remove the level parameters from kvm_arch_get_registers and kvm_arch_put_registers.
>>
>> 2. Add a new bitmap parameter to kvm_arch_get_registers and kvm_arch_put_registers.
>
> I would combine these into "replace levels with bitmap".
>
>> 3. Define a bit that correlates to our current notion of "all runtime registers".  This bit, and all bits in this bitmap, would be architecture specific.
>
> Why would that bit be architecture specific? "All runtime registers" == "registers that gdb can access" IIRC. The implementation on what exactly that means obviously is architecture specific, but the bit itself would not be, as the gdbstub wants to be able to synchronize in arch independent code.
>

How do we want to define these bits?  is it logical to break up the 
registers into smaller categories and then use masks to create 
RUNTIME_STATE, FULL_STATE, RESET_STATE?  If so, how should we define 
them?  Would they be arch specific and then we'd create the _STATE masks 
for each architecture?

If we do simply define a bit for each of the above three states instead, 
they should probably be 100% mutually exclusive to provide the best 
protection against complicated data synchronization issues (like the 
original 7/7 patch was trying to prevent).  Also, if we can assume 100% 
mutual exclusion the sync logic becomes trivial:

static void do_kvm_cpu_synchronize_state(void *arg)
{
     struct kvm_cpu_syncstate_args *args = arg;

     /* Do not sync regs that are already dirty */
     int regs_to_get = args->regmap & ~cpu->kvm_vcpu_dirty;

     kvm_arch_get_registers(args->cpu, regs_to_get);
     args->cpu->kvm_vcpu_dirty |= regs_to_get;
}

Thoughts?



-- 
-- Jason J. Herne (jjherne@linux.vnet.ibm.com)