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

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Date:   Fri, 5 Jun 2020 17:39:54 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Anchal Agarwal <anchalag@...zon.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 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

--- 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.


> 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.


-boris