linux-kernel - Re: [PATCH v1 2/8] x86/xen: simplify xen_oldmem_pfn_is

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Date:   Wed, 29 Sep 2021 10:22:07 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Dave Young <dyoung@...hat.com>, Baoquan He <bhe@...hat.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Oscar Salvador <osalvador@...e.de>,
        Mike Rapoport <rppt@...nel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>, x86@...nel.org,
        xen-devel@...ts.xenproject.org,
        virtualization@...ts.linux-foundation.org,
        kexec@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v1 2/8] x86/xen: simplify xen_oldmem_pfn_is_ram()


On 9/29/21 5:03 AM, David Hildenbrand wrote:
> On 29.09.21 10:45, David Hildenbrand wrote:
>>>
>> Can we go one step further and do
>>
>>
>> @@ -20,24 +20,11 @@ static int xen_oldmem_pfn_is_ram(unsigned long pfn)
>>           struct xen_hvm_get_mem_type a = {
>>                   .domid = DOMID_SELF,
>>                   .pfn = pfn,
>> +               .mem_type = HVMMEM_ram_rw,
>>           };
>> -       int ram;
>>    -       if (HYPERVISOR_hvm_op(HVMOP_get_mem_type, &a))
>> -               return -ENXIO;
>> -
>> -       switch (a.mem_type) {
>> -       case HVMMEM_mmio_dm:
>> -               ram = 0;
>> -               break;
>> -       case HVMMEM_ram_rw:
>> -       case HVMMEM_ram_ro:
>> -       default:
>> -               ram = 1;
>> -               break;
>> -       }
>> -
>> -       return ram;
>> +       HYPERVISOR_hvm_op(HVMOP_get_mem_type, &a);
>> +       return a.mem_type != HVMMEM_mmio_dm;


I was actually thinking of asking you to add another patch with pr_warn_once() here (and print error code as well). This call failing is indication of something going quite wrong and it would be good to know about this.


>>    }
>>    #endif
>>
>>
>> Assuming that if HYPERVISOR_hvm_op() fails that
>> .mem_type is not set to HVMMEM_mmio_dm.


I don't think we can assume that argument described as OUT in the ABI will not be clobbered in case of error


>>
>
> Okay we can't, due to "__must_check" ...


so this is a good thing ;-)


-boris