linux-kernel - Re: [PATCH v4 0/2] System Generation ID driver and VMGENID backend

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Date:   Wed, 27 Jan 2021 07:47:17 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     "Catangiu, Adrian Costin" <acatan@...zon.com>
Cc:     "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
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Subject: Re: [PATCH v4 0/2] System Generation ID driver and VMGENID backend

On Thu, Jan 21, 2021 at 10:28:16AM +0000, Catangiu, Adrian Costin wrote:
> On 12/01/2021, 14:49, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> 
>     On Tue, Jan 12, 2021 at 02:15:58PM +0200, Adrian Catangiu wrote:
>     > The first patch in the set implements a device driver which exposes a
>     > read-only device /dev/sysgenid to userspace, which contains a
>     > monotonically increasing u32 generation counter. Libraries and
>     > applications are expected to open() the device, and then call read()
>     > which blocks until the SysGenId changes. Following an update, read()
>     > calls no longer block until the application acknowledges the new
>     > SysGenId by write()ing it back to the device. Non-blocking read() calls
>     > return EAGAIN when there is no new SysGenId available. Alternatively,
>     > libraries can mmap() the device to get a single shared page which
>     > contains the latest SysGenId at offset 0.
> 
>     Looking at some specifications, the gen ID might actually be located
>     at an arbitrary address. How about instead of hard-coding the offset,
>     we expose it e.g. in sysfs?
> 
> The functionality is split between SysGenID which exposes an internal u32
> counter to userspace, and an (optional) VmGenID backend which drives
> SysGenID generation changes based on hw vmgenid updates.
> 
> The hw UUID you're referring to (vmgenid) is not mmap-ed to userspace or
> otherwise exposed to userspace. It is only used internally by the vmgenid
> driver to find out about VM generation changes and drive the more generic
> SysGenID.
> 
> The SysGenID u32 monotonic increasing counter is the one that is mmaped to
> userspace, but it is a software counter. I don't see any value in using a dynamic
> offset in the mmaped page. Offset 0 is fast and easy and most importantly it is
> static so no need to dynamically calculate or find it at runtime.

Well you are burning a whole page on it, using an offset the page
can be shared with other functionality.

> Thanks,
> Adrian.
> 
> 
> 
> 
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