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Re: [Qemu-devel] [PATCH v3 0/2] Introduce default ram size in MachineCla

From: Markus Armbruster
Subject: Re: [Qemu-devel] [PATCH v3 0/2] Introduce default ram size in MachineClass
Date: Thu, 05 Mar 2015 13:19:58 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

Nikunj A Dadhania <address@hidden> writes:

> Markus Armbruster <address@hidden> writes:
>> Nikunj A Dadhania <address@hidden> writes:
>>> Current DEFAULT_RAM_SIZE(128MB) enforced by QEMU would not work for
>>> all machines. Introduce a default_ram_size as part of MachineClass.
>>> The below patches has following behaviour:
>>> 1) If the user does not provide "-m" option, machine's default ram
>>>    size will be picked.
>>> 2) In case the user provides memory that is lesser than machine's
>>>    default ram size, we upscale the ram_size to machine's
>>>    default_ram_size. A warning is displayed for the change that qemu
>>>    has done.
>> Please do not "improve" the user's explicit order that way.  Either
>> execute the order as is, or reject it as invalid.
> If there is consensus for doing this, I can change the patches
> accordingly.
> Rejection is also change of behaviour. Because till now, a VM would
> start with any memory size, even if it's less that 128MB
> (default_ram_size). With rejection, all those VMs would fail booting
> displaying the warning. Is this OK?

I'd stick to "don't reject".  Yes, the failure mode is ugly.  But
protecting the user from it is also somewhat problematic, because we
don't generally know how much RAM the actual guest requires, and it's an
incompatible change.  Seems not worth it.

Back in 2012, we discussed rejecting RAM size less than 1MiB for PC
machines, because SeaBIOS requires at least that much, and decided
against it.  See

If you want to pursue "reject" anyway, please make sure to split this
into two separate patches: one patch to make the default ram size
machine-specific, and another patch to reject user requests for less.


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