linux-kernel - Re: [PATCH v2] x86/Hyper-V: Support for free page reporting

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Date:   Wed, 6 Jan 2021 15:35:23 +0000
From:   Wei Liu <wei.liu@...nel.org>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Sunil Muthuswamy <sunilmut@...rosoft.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Matheus Castello <matheus@...tello.eng.br>,
        KY Srinivasan <kys@...rosoft.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <liuwe@...rosoft.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>
Subject: Re: [PATCH v2] x86/Hyper-V: Support for free page reporting

On Wed, Jan 06, 2021 at 02:28:20PM +0100, Vitaly Kuznetsov wrote:
[...]
> 
> > +bool hv_query_ext_cap(u64 cap_query)
> > +{
> > +	u64 *cap;
> > +	unsigned long flags;
> > +	u64 ext_cap = 0;
> > +
> > +	/*
> > +	 * Querying extended capabilities is an extended hypercall. Check if the
> > +	 * partition supports extended hypercall, first.
> > +	 */
> > +	if (!(ms_hyperv.priv_high & HV_ENABLE_EXTENDED_HYPERCALLS))
> > +		return 0;
> > +
> > +	/*
> > +	 * Repurpose the input page arg to accept output from Hyper-V for
> > +	 * now because this is the only call that needs output from the
> > +	 * hypervisor. It should be fixed properly by introducing an
> > +	 * output arg once we have more places that require output.
> > +	 */
> 
> I remember there was a patch from Wei (realter to running Linux as root
> partition) doing the job, we can probably merge it early to avoid this
> re-purposing.
> 

We want to be frugal regarding memory usage, so in my patch the output
page is only allocated when Linux is running as the root partition.

This patch is mostly only useful when Linux is running as a child
partition. This is a different use case.

Wei.