linux-kernel - Re: [RFC PATCH v2 1/2] xen/pvh: Add memory map pointer to hvm_start

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Message-ID: <93cafa4e-9053-119e-b4e2-1a1500539409@redhat.com>
Date:   Tue, 12 Dec 2017 10:54:22 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jan Beulich <JBeulich@...e.com>,
        Maran Wilson <maran.wilson@...cle.com>
Cc:     andrew.cooper3@...rix.com, roger.pau@...rix.com, hch@...radead.org,
        x86@...nel.org, tglx@...utronix.de, xen-devel@...ts.xenproject.org,
        boris.ostrovsky@...cle.com, mingo@...hat.com, rkrcmar@...hat.com,
        Juergen Gross <jgross@...e.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, hpa@...or.com
Subject: Re: [RFC PATCH v2 1/2] xen/pvh: Add memory map pointer to
 hvm_start_info struct

On 12/12/2017 09:06, Jan Beulich wrote:
>>>> On 11.12.17 at 22:59, <pbonzini@...hat.com> wrote:
>> On 08/12/2017 09:49, Jan Beulich wrote:
>>>> + * The layout of each entry in the memory map table is as follows and no
>>>> + * padding is used between entries in the array:
>>>> + *
>>>> + *  0 +----------------+
>>>> + *    | addr           | Base address
>>>> + *  8 +----------------+
>>>> + *    | size           | Size of mapping
>>>> + * 16 +----------------+
>>>> + *    | type           | E820_TYPE_xxx
>>>> + * 20 +----------------|
>>> I'm not convinced of re-using E820 types here. I can see that this
>>> might ease the consumption in Linux, but I don't think there should
>>> be any connection to x86 aspects here - the data being supplied is
>>> x86-agnostic, and Linux'es placement of the header is also making
>>> no connection to x86 (oddly enough, the current placement in the
>>> Xen tree does, for a reason which escapes me).
>>
>> FWIW, e820 types are now part of the ACPI standard.  So using them is
>> not necessarily related to x86, and reasonably x86-agnostic.
> 
> Sort of - the description of it starts with "This interface is used in
> real mode only on IA-PC-based systems ..."

Note I said the e820 *types*.  While the interface is there for PC
compatibility, the ACPI address range types (AddressRangeMemory,
AddressRangeReserved, AddressRangeACPI, etc.) are exactly the e820 types.

> But it being there is useful in another way: It shows that there's
> an optional field making the full structure 64-bit aligned again. (It
> at the same time shows - I admit I had forgotten about this aspect -
> that the structure size isn't fixed in the first place, so consumers
> have to convert [truncate/extend] the output to their internal
> representation anyway, and hence there's even less of a reason
> to tie the proposed structure's layout to the E820 one.)

My point was that the e820 types are okay to use in an
architecture-agnostic way in my opinion.  The layout only matters so
much, as there aren't many ways to encode a memory map (note I do agree
about that alignment dword).

Paolo