Re: [RFC PATCH 2/2] Add HvExtCallGetBootZeroedMemory

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Apr 20, 2026 at 10:25:43AM +0100, Florian Schmidt wrote:
> Hi Marcelo,
> 
> On 2026-04-17 06:51, Marcelo Tosatti wrote:
> > Is this to improve boot time of very large guests ?
> 
> It depends a bit on how you define "boot time". It won't make a difference
> if you measure the time it takes to create the VM until you start the vCPUs.
> It may even make a huge difference if you measure the time until a user can
> first log in.
> 
> What it will do is reduce the time the guest spends on writing memory,
> mostly in the background, to zero out all pages it has gotten from the
> hypervisor. The main advantage (other than saving CPU cycles that are
> effectively wasted on an unneeded operation) is that if you don't
> pre-allocate all the VMs memory, then the VM won't touch all pages after
> boot, keeping the RSS down until the memory is actually used. That's for
> example useful if you overcommit/oversubscribe VM memory.
> 
> Cheers,
> Florian

Hi Florian,

That is useful!