Re: [patch] KVM: paravirtual guest support

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Ingo Molnar wrote:
> Subject: [patch] KVM: paravirtual guest support
> From: Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org>
>
> this enables a CONFIG_PARAVIRT Linux guest kernel to establish a 
> hypercall API to a KVM host. If successfully registered, then the Linux 
> guest will optimize a few things like its interrupt controller, io-delay 
> and it also registers its cr3-cache structures with the host. (but the 
> host will not touch those, just yet)
>
> (this is fully backwards compatible - if the WRMSR fails then the Linux
> guest continues to execute as a native kernel.)
>
> Signed-off-by: Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org>
> ---
> +			/*
> +			 * Cache-hit: we load the cached host-CR3 value.
> +			 * This never causes any VM exit. (if it does then the
> +			 * hypervisor could do nothing with this instruction
> +			 * and the guest OS would be aborted)
> +			 */
> +			asm volatile("movl %0, %%cr3"
> +				: : "r" (cache->entry[idx].host_cr3));
> +			goto out;
> +		}
> +	}
> +
> +	/*
> +	 * Cache-miss. Load the guest-cr3 value into cr3, which will
> +	 * cause a VM exit to the hypervisor, which then loads the
> +	 * host cr3 value and updates the cr3_cache.
> +	 */
> +	asm volatile("movl %0, %%cr3" : : "r" (guest_cr3));
> +out:
> +	put_cpu_var(para_state);
> +}
> +
>   

There's a problem here.  A cache-hit cr3 is an hpa, while a cache-miss 
cr3 is a gpa.  The two could alias:

/* pseudoassembler */

guest:
  mov $0x1234, %cr3 /* cache miss */

host:
  creates shadow for 0x1234 at 0x5678
  set cr3 cache entry: $0x1234 -> 0x5678

guest:
  mov $0x5678, %cr3 /* cache miss */
  vmx accepts that as a cached cr3 equivalent to the guest's 0x1234, 
whereas a 0x5678 gpa cr3 was intended.

The only solution I see is to use the hypercall API on a cache miss.  
Once the guest indicates it wants to use the cr3 cache, set_cr3() should 
fault, and cr3 should only be accepted from the hypercall.

-- 
error compiling committee.c: too many arguments to function


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