Re: [pve-devel] [PATCH docs 1/2] pci passthrough: mention incompatibility with ballooning

[Friedrich Weber <f.weber@proxmox.com>]

I took another look at ballooning+PCI passthrough and reporting of
memory usage with Markus. Apparently QEMU does not *always* map the
complete guest memory -- at least it didn't with a passed-through NIC.
So both the warning as well as the docs section may be worded too
strongly ("Ballooning is not possible", "VM will use maximum configured
memory", "QEMU needs to map" ...). I'll have to take another look at
this to see how we can phrase this correctly (and hopefully somewhat
precisely) for v2.

On 14/11/2023 11:20, Friedrich Weber wrote:
> On 14/11/2023 09:30, Fiona Ebner wrote:
>> Am 13.11.23 um 18:09 schrieb Friedrich Weber:
>>>  
>>> +xref:qm_ballooning[Automatic memory allocation (ballooning)] is not possible
>>> +when using PCI(e) passthrough. As the PCI device may use DMA (Direct Memory
>>> +Access), QEMU needs to map the complete guest memory on VM startup. Hence, the
>>> +QEMU process will use at least the (maximum) configured amount of VM memory and
>>> +setting a minimum amount does not have any effect. When using PCI(e)
>>> +passthrough, it is recommended to set memory and minimum memory to the same
>>> +amount and keep the balloning device enabled. However, keep in mind that the
>>
>> typo: s/balloning/ballooning/
> 
> Oops, thanks.
> 
>> Is there any advantage to keeping the ballooning device enabled?
>>
>>> +memory consumption reported in the GUI for the VM may be much lower than the
>>> +memory consumption of the QEMU process.
>>
>> Maybe mention what the reported value is?
> 
> In my understanding: If the ballooning device is enabled (and a balloon
> driver present in the guest), the VM memory usage numbers are taken as
> reported by the balloon driver [1] (I'd say "as seen from within the
> guest"?). If the ballooning device is disabled, they are inferred from
> the rss and vsize of the QEMU process [2], so I'd say "as seen from the
> host".
> 
> I guess in the end the user has to decide which perspective they care
> about? I'll try to make this clearer in the v2.
> 
> [1]
> https://git.proxmox.com/?p=qemu-server.git;a=blob;f=PVE/QemuServer.pm;h=c465fb6f64ae30dec5112fc4439f9181c2eba4e9;hb=feb51881d#l3013
> [2]
> https://git.proxmox.com/?p=qemu-server.git;a=blob;f=PVE/QemuServer.pm;h=c465fb6f64ae30dec5112fc4439f9181c2eba4e9;hb=feb51881d#l2967
> 
> 
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