kvm_regs vs kvm_sregs?

kvm_regs vs kvm_sregs?

[Hollis Blanchard]

What is the distinction between kvm_regs and kvm_sregs? As far as I can
see, kvm_regs is only used when emulating IO, emulating MMIO, and
emulating CPUID, where guest GPRs are directly modified. kvm_sregs is
only used for full CPU state save (for later restore).

When the kernel had to use copy_to_user() to transfer state to userland,
I can see that this split resulted in less memory copying. However, now
that userland can directly map register state without a copy, why not
combine the two structures?

I guess it takes longer to copy state out of the VMCS into kvm_sregs, so
why bother if userspace isn't going to use it?

-Hollis


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Re: kvm_regs vs kvm_sregs?

[Avi Kivity]

Hollis Blanchard wrote:
> What is the distinction between kvm_regs and kvm_sregs? As far as I can
> see, kvm_regs is only used when emulating IO, emulating MMIO, and
> emulating CPUID, where guest GPRs are directly modified. kvm_sregs is
> only used for full CPU state save (for later restore).
>   

After the userspace interface changes, neither of them are used except 
for migration, savevm/loadvm, and debugging. Theoretically they could be 
unified, practically SET_SREGS is a dangerous operation on Intel cpus 
due to kvm's very imperfect real mode support.

> When the kernel had to use copy_to_user() to transfer state to userland,
> I can see that this split resulted in less memory copying. However, now
> that userland can directly map register state without a copy, why not
> combine the two structures?
>   

We don't map the register state as that would require us to sync it on 
every exit.

> I guess it takes longer to copy state out of the VMCS into kvm_sregs, so
> why bother if userspace isn't going to use it?
>   

When it's needed, it's really needed.  Or did I misunderstand the question?

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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Re: kvm_regs vs kvm_sregs?

[Hollis Blanchard]

On Sun, 2007-03-18 at 07:13 +0200, Avi Kivity wrote:
> Hollis Blanchard wrote:
> > What is the distinction between kvm_regs and kvm_sregs? As far as I can
> > see, kvm_regs is only used when emulating IO, emulating MMIO, and
> > emulating CPUID, where guest GPRs are directly modified. kvm_sregs is
> > only used for full CPU state save (for later restore).
> >   
> 
> After the userspace interface changes, neither of them are used except 
> for migration, savevm/loadvm, and debugging. Theoretically they could be 
> unified, practically SET_SREGS is a dangerous operation on Intel cpus 
> due to kvm's very imperfect real mode support.
> 
> > When the kernel had to use copy_to_user() to transfer state to userland,
> > I can see that this split resulted in less memory copying. However, now
> > that userland can directly map register state without a copy, why not
> > combine the two structures?
> >   
> 
> We don't map the register state as that would require us to sync it on 
> every exit.
> 
> > I guess it takes longer to copy state out of the VMCS into kvm_sregs, so
> > why bother if userspace isn't going to use it?
> >   
> 
> When it's needed, it's really needed.  Or did I misunderstand the question?

I guess I misunderstood. I thought the mmap was for userspace to access
kvm_regs, which previously was needed when emulating IO.

Apparently this is no longer the case: instead the data is copied
somewhere into mmap space and then complete_pio() is responsible for
loading that raw data into the guest register state.

Seems a bit more convoluted, but I guess you've managed to divorce user
and kernel data structures, if that was the goal.

-Hollis


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