On Mon, May 27, 2019 at 06:39:53PM +0200, Vitaly Kuznetsov wrote:
> Roman Kagan <rka...@virtuozzo.com> writes:
> > On Fri, May 17, 2019 at 04:19:17PM +0200, Vitaly Kuznetsov wrote:
> >> +static struct kvm_cpuid2 *try_get_hv_cpuid(CPUState *cs, int max)
> >> +{
> >> + struct kvm_cpuid2 *cpuid;
> >> + int r, size;
> >> +
> >> + size = sizeof(*cpuid) + max * sizeof(*cpuid->entries);
> >> + cpuid = g_malloc0(size);
> >> + cpuid->nent = max;
> >> +
> >> + r = kvm_vcpu_ioctl(cs, KVM_GET_SUPPORTED_HV_CPUID, cpuid);
> >> + if (r == 0 && cpuid->nent >= max) {
> >> + r = -E2BIG;
> >> + }
> >> + if (r < 0) {
> >> + if (r == -E2BIG) {
> >> + g_free(cpuid);
> >> + return NULL;
> >> + } else {
> >> + fprintf(stderr, "KVM_GET_SUPPORTED_HV_CPUID failed: %s\n",
> >> + strerror(-r));
> >> + exit(1);
> >> + }
> >> + }
> >> + return cpuid;
> >> +}
> >> +
> >> +/*
> >> + * Run KVM_GET_SUPPORTED_HV_CPUID ioctl(), allocating a buffer large
> >> enough
> >> + * for all entries.
> >> + */
> >> +static struct kvm_cpuid2 *get_supported_hv_cpuid(CPUState *cs)
> >> +{
> >> + struct kvm_cpuid2 *cpuid;
> >> + int max = 7; /* 0x40000000..0x40000005, 0x4000000A */
> >> +
> >> + /*
> >> + * When the buffer is too small, KVM_GET_SUPPORTED_HV_CPUID fails with
> >> + * -E2BIG, however, it doesn't report back the right size. Keep
> >> increasing
> >> + * it and re-trying until we succeed.
> >> + */
> >
> > I'm still missing the idea of reiterating more than once: the ioctl
> > returns the actual size of the array.
>
> Hm, I think I checked that and it doesn't seem to be the case.
>
> The code in kvm_vcpu_ioctl_get_hv_cpuid():
>
> if (cpuid->nent < nent)
> return -E2BIG;
>
> if (cpuid->nent > nent)
> cpuid->nent = nent;
>
> (I think I even ran a test after your comment on v1 and it it
> confirmed nent is not set on E2BIG). Am I missing something obvious?
Indeed, I saw kvm_vcpu_ioctl_get_cpuid2() always setting ->nent on
return and assumed so did kvm_vcpu_ioctl_get_hv_cpuid(). I stand
corrected, please disregard this comment.
(What was the reason for not following this pattern in
kvm_vcpu_ioctl_get_hv_cpuid BTW?)
Thanks, and sorry for confusion,
Roman.
