linux-kernel - Re: [PATCH 03/12] x86/xen: Introduce new function to map HYPERVISOR_shared

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Date:   Mon, 8 Jun 2020 16:52:35 +0000
From:   Anchal Agarwal <anchalag@...zon.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
CC:     <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
        <hpa@...or.com>, <x86@...nel.org>, <jgross@...e.com>,
        <linux-pm@...r.kernel.org>, <linux-mm@...ck.org>,
        <kamatam@...zon.com>, <sstabellini@...nel.org>,
        <konrad.wilk@...cle.com>, <roger.pau@...rix.com>,
        <axboe@...nel.dk>, <davem@...emloft.net>, <rjw@...ysocki.net>,
        <len.brown@...el.com>, <pavel@....cz>, <peterz@...radead.org>,
        <eduval@...zon.com>, <sblbir@...zon.com>,
        <xen-devel@...ts.xenproject.org>, <vkuznets@...hat.com>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <dwmw@...zon.co.uk>, <benh@...nel.crashing.org>
Subject: Re: [PATCH 03/12] x86/xen: Introduce new function to map
 HYPERVISOR_shared_info on Resume

On Fri, Jun 05, 2020 at 05:39:54PM -0400, Boris Ostrovsky wrote:
> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
> 
> 
> 
> On 6/4/20 7:03 PM, Anchal Agarwal wrote:
> > On Sat, May 30, 2020 at 07:02:01PM -0400, Boris Ostrovsky wrote:
> >> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
> >>
> >>
> >>
> >> On 5/19/20 7:25 PM, Anchal Agarwal wrote:
> >>> Introduce a small function which re-uses shared page's PA allocated
> >>> during guest initialization time in reserve_shared_info() and not
> >>> allocate new page during resume flow.
> >>> It also  does the mapping of shared_info_page by calling
> >>> xen_hvm_init_shared_info() to use the function.
> >>>
> >>> Signed-off-by: Anchal Agarwal <anchalag@...zon.com>
> >>> ---
> >>>  arch/x86/xen/enlighten_hvm.c | 7 +++++++
> >>>  arch/x86/xen/xen-ops.h       | 1 +
> >>>  2 files changed, 8 insertions(+)
> >>>
> >>> diff --git a/arch/x86/xen/enlighten_hvm.c b/arch/x86/xen/enlighten_hvm.c
> >>> index e138f7de52d2..75b1ec7a0fcd 100644
> >>> --- a/arch/x86/xen/enlighten_hvm.c
> >>> +++ b/arch/x86/xen/enlighten_hvm.c
> >>> @@ -27,6 +27,13 @@
> >>>
> >>>  static unsigned long shared_info_pfn;
> >>>
> >>> +void xen_hvm_map_shared_info(void)
> >>> +{
> >>> +     xen_hvm_init_shared_info();
> >>> +     if (shared_info_pfn)
> >>> +             HYPERVISOR_shared_info = __va(PFN_PHYS(shared_info_pfn));
> >>> +}
> >>> +
> >>
> >> AFAICT it is only called once so I don't see a need for new routine.
> >>
> >>
> > HYPERVISOR_shared_info can only be mapped in this scope without refactoring
> > much of the code.
> 
> 
> Refactoring what? All am suggesting is
>
shared_info_pfn does not seem to be in scope here, it's scope is limited
to enlighten_hvm.c. That's the reason I introduced a new function there.

> --- a/arch/x86/xen/suspend.c
> +++ b/arch/x86/xen/suspend.c
> @@ -124,7 +124,9 @@ static void xen_syscore_resume(void)
>                 return;
> 
>         /* No need to setup vcpu_info as it's already moved off */
> -       xen_hvm_map_shared_info();
> +       xen_hvm_init_shared_info();
> +       if (shared_info_pfn)
> +               HYPERVISOR_shared_info = __va(PFN_PHYS(shared_info_pfn));
> 
>         pvclock_resume();
> 
> >> And is it possible for shared_info_pfn to be NULL in resume path (which
> >> is where this is called)?
> >>
> >>
> > I don't think it should be, still a sanity check but I don't think its needed there
> > because hibernation will fail in any case if thats the case.
> 
> 
> If shared_info_pfn is NULL you'd have problems long before hibernation
> started. We set it in xen_hvm_guest_init() and never touch again.
> 
> 
> In fact, I'd argue that it should be __ro_after_init.
> 
> 
I agree, and I should have mentioned that I will remove that check and its not
necessary as this gets mapped way early in the boot process.
> > However, HYPERVISOR_shared_info does needs to be re-mapped on resume as its been
> > marked to dummy address on suspend. Its also safe in case va changes.
> > Does the answer your question?
> 
> 
> I wasn't arguing whether HYPERVISOR_shared_info needs to be set, I was
> only saying that shared_info_pfn doesn't need to be tested.
> 
Got it. :)
> 
> -boris
> 
Thanks,
Anchal
>