linux-kernel - Re: [kvm-devel] [PATCH 3/3] KVM: Expose MSRs to userspace

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Message-Id: <200611162008.48931.arnd@arndb.de>
Date:	Thu, 16 Nov 2006 20:08:48 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	kvm-devel@...ts.sourceforge.net
Cc:	Avi Kivity <avi@...ranet.com>, akpm@...l.org,
	linux-kernel@...r.kernel.org, uril@...ranet.com
Subject: Re: [kvm-devel] [PATCH 3/3] KVM: Expose MSRs to userspace

On Thursday 16 November 2006 19:04, Avi Kivity wrote:
> +struct kvm_msr_entry {
> +       __u32 index;
> +       __u32 reserved;
> +       __u64 data;
> +};
> +
> +/* for KVM_GET_MSRS and KVM_SET_MSRS */
> +struct kvm_msrs {
> +       __u32 vcpu;
> +       __u32 nmsrs; /* number of msrs in entries */
> +
> +       union {
> +               struct kvm_msr_entry __user *entries;
> +               __u64 padding;
> +       };
> +};

ioctl interfaces with pointers in them are generally a bad idea,
though you handle most of the points against them fine here
(endianess doesn't matter, padding is correct).

Still, it might be better not to set a bad example. Is accessing
the MSRs actually performance critical? If not, you could
define the ioctl to take only a single entry argument.

A possible alternative could also be to have a variable length
argument like below, but that creates other problems:

+struct kvm_msrs {
+       __u32 vcpu;
+       __u32 nmsrs; /* number of msrs in entries */
+       struct kvm_msr_entry entries[0]; /* followed by actual msrs */
+};

This would mean that you can't tell the transfer size from the
ioctl number, but you can't do that in your code either, because
you do two separate transfers.

	Arnd <><
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