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SubjectRe: [PATCH 3/6] x86: Add nopv parameter to disable PV extensions
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On 23.06.19 15:01, Zhenzhong Duan wrote:
> In virtualization environment, PV extensions (drivers, interrupts,
> timers, etc) are enabled in the majority of use cases which is the
> best option.
>
> However, in some cases (kexec not fully working, benchmarking)
> we want to disable PV extensions. As such introduce the
> 'nopv' parameter that will do it.
>
> There is already 'xen_nopv' parameter for XEN platform but not for
> others. 'xen_nopv' can then be removed with this change.
>
> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
> Cc: xen-devel@lists.xenproject.org
> ---
> Documentation/admin-guide/kernel-parameters.txt | 4 ++++
> arch/x86/kernel/cpu/hypervisor.c | 11 +++++++++++
> 2 files changed, 15 insertions(+)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 138f666..b352f36 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5268,6 +5268,10 @@
> improve timer resolution at the expense of processing
> more timer interrupts.
>
> + nopv= [X86]
> + Disables the PV optimizations forcing the guest to run
> + as generic guest with no PV drivers.
> +
> xirc2ps_cs= [NET,PCMCIA]
> Format:
> <irq>,<irq_mask>,<io>,<full_duplex>,<do_sound>,<lockup_hack>[,<irq2>[,<irq3>[,<irq4>]]]
> diff --git a/arch/x86/kernel/cpu/hypervisor.c b/arch/x86/kernel/cpu/hypervisor.c
> index 479ca47..4f2c875 100644
> --- a/arch/x86/kernel/cpu/hypervisor.c
> +++ b/arch/x86/kernel/cpu/hypervisor.c
> @@ -85,10 +85,21 @@ static void __init copy_array(const void *src, void *target, unsigned int size)
> to[i] = from[i];
> }
>
> +static bool nopv;
> +static __init int xen_parse_nopv(char *arg)
> +{
> + nopv = true;
> + return 0;
> +}
> +early_param("nopv", xen_parse_nopv);
> +
> void __init init_hypervisor_platform(void)
> {
> const struct hypervisor_x86 *h;
>
> + if (nopv)
> + return;
> +

Oh, this is no good idea.

There are guest types which just won't work without pv interfaces, like
Xen PV and Xen PVH. Letting them fail due to just a wrong command line
parameter is not nice, especially as the failure might be very hard to
track down to the issue for the user.

I guess you could add a "ignore_nopv" member to struct hypervisor_x86
set to true for the mentioned guest types and call the detect functions
only if nopv is false or ignore_nopv is true.


Juergen

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