linux-kernel - Re: [PATCH v2 08/17] x86/hyperv: handling hypercall page setup for root

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Message-ID: <20201113153333.yt54enp5dbqjj5nu@liuwe-devbox-debian-v2>
Date:   Fri, 13 Nov 2020 15:33:33 +0000
From:   Wei Liu <wei.liu@...nel.org>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Wei Liu <wei.liu@...nel.org>,
        Linux on Hyper-V List <linux-hyperv@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        Linux Kernel List <linux-kernel@...r.kernel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Sunil Muthuswamy <sunilmut@...rosoft.com>,
        Nuno Das Neves <nunodasneves@...ux.microsoft.com>,
        Lillian Grassin-Drake <ligrassi@...rosoft.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 08/17] x86/hyperv: handling hypercall page setup for
 root

On Thu, Nov 12, 2020 at 04:51:09PM +0100, Vitaly Kuznetsov wrote:
> Wei Liu <wei.liu@...nel.org> writes:
> 
> > When Linux is running as the root partition, the hypercall page will
> > have already been setup by Hyper-V. Copy the content over to the
> > allocated page.
> >
> > The suspend, resume and cleanup paths remain untouched because they are
> > not supported in this setup yet.
> 
> What about adding BUG_ONs there then?

I generally avoid cluttering code if I'm sure it definitely does not
work.

In any case, adding BUG_ONs is not the right answer. Both hv_suspend and
hv_resume can return an error code. I would rather just do

   if (hv_root_partition)
       return -EPERM;

in both places.

And also make hv_is_hibernation_supported return false when Linux is the
root partition.

> > +
> > +	if (hv_root_partition) {
> > +		struct page *pg;
> > +		void *src, *dst;
> > +
> > +		/*
> > +		 * For the root partition, the hypervisor will set up its
> > +		 * hypercall page. The hypervisor guarantees it will not show
> > +		 * up in the root's address space. The root can't change the
> > +		 * location of the hypercall page.
> > +		 *
> > +		 * Order is important here. We must enable the hypercall page
> > +		 * so it is populated with code, then copy the code to an
> > +		 * executable page.
> > +		 */
> > +		wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> > +
> > +		pg = vmalloc_to_page(hv_hypercall_pg);
> > +		dst = kmap(pg);
> > +		src = memremap(hypercall_msr.guest_physical_address << PAGE_SHIFT, PAGE_SIZE,
> > +				MEMREMAP_WB);
> > +		BUG_ON(!(src && dst));
> > +		memcpy(dst, src, PAGE_SIZE);
> 
> Super-nit: while on x86 PAGE_SIZE always matches HV_HYP_PAGE_SIZE, would
> it be more accurate to use the later here?

Sure. That can be done.

Wei.