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bh=pT1nQqivj0up9gyWl0CfF5tqqDD1dzEKO4x1O3sJYZo=; b=fpw62yqr8Jas7JgBzctZgnEtla tHOy015Cu2XVfck31a0bMjnfNOr6aalRRCaUoqGL+nsTLnd/myWawT1ADG3O/SC0pzMgBT14Ea5l5 5bkquThrHRHrYYTVmdE6yuErkRjQEqFgAo4bBvEhnluh/qOipqHLgqlXjLG5guC53i6E=; Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.89) (envelope-from ) id 1jI8mk-0007UM-H3; Sat, 28 Mar 2020 10:33:50 +0000 Received: from [54.239.6.185] (helo=a483e7b01a66.ant.amazon.com) by xenbits.xenproject.org with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.89) (envelope-from ) id 1jI8mk-0005tO-3q; Sat, 28 Mar 2020 10:33:50 +0000 To: Jan Beulich References: <20200322161418.31606-1-julien@xen.org> <20200322161418.31606-5-julien@xen.org> From: Julien Grall Message-ID: <5d554850-34eb-5ff7-6904-a304e444f4a6@xen.org> Date: Sat, 28 Mar 2020 10:33:47 +0000 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 8bit Subject: Re: [Xen-devel] [PATCH 04/17] xen: Convert virt_to_mfn() and mfn_to_virt() to use typesafe MFN X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: Stefano Stabellini , Wei Liu , Konrad Rzeszutek Wilk , Andrew Cooper , Julien Grall , Ian Jackson , George Dunlap , Ross Lagerwall , Lukasz Hawrylko , xen-devel@lists.xenproject.org, Volodymyr Babchuk , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" Hi, On 26/03/2020 09:09, Jan Beulich wrote: > On 25.03.2020 19:21, Julien Grall wrote: >> On 25/03/2020 15:27, Jan Beulich wrote: >>> On 22.03.2020 17:14, julien@xen.org wrote: >>>> @@ -785,21 +781,21 @@ bool is_iomem_page(mfn_t mfn) >>>>       return (page_get_owner(page) == dom_io); >>>>   } >>>>   -static int update_xen_mappings(unsigned long mfn, unsigned int cacheattr) >>>> +static int update_xen_mappings(mfn_t mfn, unsigned int cacheattr) >>>>   { >>>>       int err = 0; >>>> -    bool alias = mfn >= PFN_DOWN(xen_phys_start) && >>>> -         mfn < PFN_UP(xen_phys_start + xen_virt_end - XEN_VIRT_START); >>>> +    bool alias = mfn_x(mfn) >= PFN_DOWN(xen_phys_start) && >>>> +         mfn_x(mfn) < PFN_UP(xen_phys_start + xen_virt_end - XEN_VIRT_START); >>>>       unsigned long xen_va = >>>> -        XEN_VIRT_START + ((mfn - PFN_DOWN(xen_phys_start)) << PAGE_SHIFT); >>>> +        XEN_VIRT_START + mfn_to_maddr(mfn_add(mfn, -PFN_DOWN(xen_phys_start))); >>> >>> Depending on the types involved (e.g. in PFN_DOWN()) this may >>> or may not be safe, so I consider such a transformation at >>> least fragile. I think we either want to gain mfn_sub() or >>> keep this as a "real" subtraction. >> I want to avoid mfn_x() as much as possible when everything can >> be done using typesafe operation. But i am not sure how >> mfn_sub() would solve the problem. Do you mind providing more >> information? > > Consider PFN_DOWN() potentially returning "unsigned int". The > negation of an unsigned int is still an unsigned int, and hence > e.g. -1U (which might result here) is really 0xFFFFFFFF rather > than -1L / -1UL as intended. Whereas with mfn_sub() the > conversion to unsigned long of the (positive) value to subtract > would occur as part of evaluating function arguments, and the > resulting subtraction would then be correct. I will have a look to introduce mfn_sub(). > >>>> @@ -584,21 +584,21 @@ static unsigned long init_node_heap(int node, unsigned long mfn, >>>>           needed = 0; >>>>       } >>>>       else if ( *use_tail && nr >= needed && >>>> -              arch_mfn_in_directmap(mfn + nr) && >>>> +              arch_mfn_in_directmap(mfn_x(mfn_add(mfn, nr))) && >>>>                 (!xenheap_bits || >>>> -               !((mfn + nr - 1) >> (xenheap_bits - PAGE_SHIFT))) ) >>>> +               !((mfn_x(mfn) + nr - 1) >> (xenheap_bits - PAGE_SHIFT))) ) >>> >>> May I suggest consistency here: This one uses +, while ... >>> >>>>       { >>>> -        _heap[node] = mfn_to_virt(mfn + nr - needed); >>>> -        avail[node] = mfn_to_virt(mfn + nr - 1) + >>>> +        _heap[node] = mfn_to_virt(mfn_add(mfn, nr - needed)); >>>> +        avail[node] = mfn_to_virt(mfn_add(mfn, nr - 1)) + >>>>                         PAGE_SIZE - sizeof(**avail) * NR_ZONES; >>>>       } >>>>       else if ( nr >= needed && >>>> -              arch_mfn_in_directmap(mfn + needed) && >>>> +              arch_mfn_in_directmap(mfn_x(mfn_add(mfn, needed))) && >>> >>> ... this one uses mfn_add() despite the mfn_x() around it, and ... >> >> So the reason I used mfn_x(mfn_add(mfn, needed)) here is I plan >> to convert arch_mfn_in_directmap() to use typesafe soon. In the >> two others cases... >> >>>>                 (!xenheap_bits || >>>> -               !((mfn + needed - 1) >> (xenheap_bits - PAGE_SHIFT))) ) >>>> +               !((mfn_x(mfn) + needed - 1) >> (xenheap_bits - PAGE_SHIFT))) ) >>> >>> ... here you use + again. My personal preference would be to avoid >>> constructs like mfn_x(mfn_add()). >> >> ... I am still unsure how to avoid mfn_x(). Do you have any ideas? > > I don't see how it can be avoided right now. But I also don't see > why - for consistency, as said - you couldn't use mfn_x() also in > the middle case. You could then still convert to mfn_add() with > that future change of yours. I could have as I could also have converted arch_mfn_in_directmap() to use typesafe MFN. Anything around typesafe is a can of worms and this is the fine line I found. Anyway, I could not be bother to bikeshed... So I going to switch the other one to mfn_x(...) + needed. > >>>> --- a/xen/include/asm-x86/mm.h >>>> +++ b/xen/include/asm-x86/mm.h >>>> @@ -667,7 +667,7 @@ static inline bool arch_mfn_in_directmap(unsigned long mfn) >>>>   { >>>>       unsigned long eva = min(DIRECTMAP_VIRT_END, HYPERVISOR_VIRT_END); >>>>   -    return mfn <= (virt_to_mfn(eva - 1) + 1); >>>> +    return mfn <= mfn_x(mfn_add(virt_to_mfn(eva - 1),  1)); >>> >>> Even if you wanted to stick to using mfn_add() here, there's one >>> blank too many after the comma. >> >> I will remove the extra blank. Regarding the construction, I have >> been wondering for a couple of years now whether we should >> introduce mfn_{lt, gt}. What do you think? > > I too have been wondering, and wouldn't mind their introduction > (plus mfn_le / mfn_ge perhaps). But it'll truly help you here > anyway only once the function parameter is also mfn_t. This is a longer term plan. So I am going to leave it like that for now until I manage to find time. Cheers, -- Julien Grall