Re: [PATCH] kvm tools: Make the whole guest memory mergeable

[Sasha Levin <levinsasha928@gmail.com>]

On Fri, 2011-12-16 at 16:36 +0800, Zang Hongyong wrote:
> 于 2011/12/16,星期五 15:23, Sasha Levin 写道:
> > On Fri, 2011-12-16 at 15:02 +0800, Zang Hongyong wrote:
> >> 于 2011/12/16,星期五 13:50, Sasha Levin 写道:
> >>> On Fri, 2011-12-16 at 09:01 +0800, zanghongyong@huawei.com wrote:
> >>>> If a guest's ram_size exceeds KVM_32BIT_GAP_START, the corresponding kvm tool's
> >>>> virtual address size should be (ram_size + KVM_32BIT_GAP_SIZE), rather than ram_size.
> >>> You're right.
> >>>
> >>> There are more places than just the madvise() code which make the same
> >>> error you've spotted (for example, the memslot allocation code), so
> >>> instead of trying to fix all of them I'd suggest to just update ram_size
> >>> in kvm__arch_init() before allocating everything - that should fix all
> >>> of them at once.
> >>>
> >> Yes. There are other scenarios with the same error.
> >> However ram_size sometimes means real guest ram size, and sometimes 
> >> means virtual address
> >> size of kvm tool's user space. Shall we define a new variable?
> > Let's keep it simple. If the user requests more than RAM than
> > KVM_32BIT_GAP_START just increase it by KVM_32BIT_GAP_SIZE, this way
> > mapped size == guest size always (we can madvise(MADV_DONTNEED) the gap
> > in the mmapped ram).
> >
> > Since a user which requests more than KVM_32BIT_GAP_START will have to
> > be on 64bit host anyway, there shouldn't be any issue with that.
> >
> Do you mean increase *kvm->ram_size* by KVM_32BIT_GAP_SIZE?
> but sometimes kvm->ram_size stands for guest physical ram size (for
> example in kvm__init_ram() code).

Yup, kvm->ram_size.

If the user requested more than KVM_32BIT_GAP_START, we pretty much have
to create the gap, so instead of playing around with different
interpretations of ram_size, lets add the gap size - this will let us
have just one ram_size.

mmap()ing extra space for the gap is free, and that was the plan in the
first place (we just got the math wrong :) ).

Do you see an issue with increasing kvm->ram_size?

-- 

Sasha.