Messages in this thread | | | Date | Thu, 16 Nov 2006 21:17:18 +0200 | From | Avi Kivity <> | Subject | Re: [kvm-devel] [PATCH 3/3] KVM: Expose MSRs to userspace |
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Arnd Bergmann wrote: > 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. > >
But then you can't dynamically determine which MSRs are available.
And no, reading/setting MSRs isn't performance critical for the current use cases.
> 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. > >
Heh. That was the original implementation by Uri. I felt that was wrong because _IOW() encodes the size in the ioctl number, bit the actual size is different.
> Arnd <>< >
-- Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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