linux-kernel - Re: [PATCH v3 5/8] export paravirtual cpuid flags in KVM_GET_SUPPORTED

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Date:	Tue, 11 May 2010 11:29:08 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	zamsden@...hat.com
Subject: Re: [PATCH v3 5/8] export paravirtual cpuid flags in KVM_GET_SUPPORTED_CPUID

On 05/06/2010 12:27 AM, Glauber Costa wrote:
> Right now, we were using individual KVM_CAP entities to communicate
> userspace about which cpuids we support. This is suboptimal, since it
> generates a delay between the feature arriving in the host, and
> being available at the guest.
>
> A much better mechanism is to list para features in KVM_GET_SUPPORTED_CPUID.
> This makes userspace automatically aware of what we provide. And if we
> ever add a new cpuid bit in the future, we have to do that again,
> which create some complexity and delay in feature adoption.
>
>
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1971,6 +1971,23 @@ static void do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>   		}
>   		break;
>   	}
> +	case KVM_CPUID_SIGNATURE: {
> +		char signature[12] = "KVMKVMKVM\0\0";
> +		u32 *sigptr = (u32 *)signature;
> +		entry->eax = 1;
>    

Don't understand where the value for eax comes from.  qemu-kvm-x86.c has 0.

> @@ -2017,6 +2034,19 @@ static int kvm_dev_ioctl_get_supported_cpuid(struct kvm_cpuid2 *cpuid,
>   	for (func = 0x80000001; func<= limit&&  nent<  cpuid->nent; ++func)
>   		do_cpuid_ent(&cpuid_entries[nent], func, 0,
>   			&nent, cpuid->nent);
> +
> +	
> +
> +	r = -E2BIG;
> +	if (nent>= cpuid->nent)
> +		goto out_free;
> +
> +	do_cpuid_ent(&cpuid_entries[nent], KVM_CPUID_SIGNATURE, 0,&nent,
> +		     cpuid->nent);
>    

Need a check here too, or the next call can overflow.  Would have been 
better to make do_cpuid_ent() check and return an error.

> +
> +	do_cpuid_ent(&cpuid_entries[nent], KVM_CPUID_FEATURES, 0,&nent,
> +		     cpuid->nent);
> +
>   	r = -E2BIG;
>   	if (nent>= cpuid->nent)
>   		goto out_free;
>    


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

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