Re: [Xen-devel] [PATCH 1 of 3] After preparing a page for page-in, allow immediate fill-in of the page contents

[Tim Deegan <tim@xxxxxxx>]

At 12:21 -0500 on 01 Dec (1322742072), Andres Lagar-Cavilla wrote:
>  xen/arch/x86/mm/mem_event.c  |   2 +-
>  xen/arch/x86/mm/mem_paging.c |   2 +-
>  xen/arch/x86/mm/p2m.c        |  32 ++++++++++++++++++++++++++++++--
>  xen/include/asm-x86/p2m.h    |   2 +-
>  xen/include/public/domctl.h  |   8 ++++++--
>  5 files changed, 39 insertions(+), 7 deletions(-)
> 
> 
> p2m_mem_paging_prep ensures that an mfn is backing the paged-out gfn, and
> transitions to the next state in the paging state machine for that page.
> Foreign mappings of the gfn will now succeed. This is the key idea, as
> it allows the pager to now map the gfn and fill in its contents.
> 
> Unfortunately, it also allows any other foreign mapper to map the gfn and read
> its contents. This is particularly dangerous when the populate is launched
> by a foreign mapper in the first place, which will be actively retrying the
> map operation and might race with the pager. Qemu-dm being a prime example.
> 
> Fix the race by allowing a buffer to be optionally passed in the prep
> operation, and having the hypervisor memcpy from that buffer into the newly
> prepped page before promoting the gfn type.
> 
> Signed-off-by: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>

Acked-by: Tim Deegan <tim@xxxxxxx>

Once Olaf OKs the xenpaging change, this whole set can go in.

Tim.

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