Re: [PATCH v5 05/15] KVM: arm64: Harvest stage-2 dirty state into the host folio account

[Leonardo Bras <leo.bras@arm.com>]

Hi Tian,
Sorry for the delay in reviewing...

On Tue, Sep 29, 2026 at 06:36:45PM +0800, Tian Zheng wrote:
> With VTCR_EL2.HD set, hardware promotes writable-clean descriptors to
> writable-dirty without any VM exit, and the only record of the write
> is the S2AP[1] bit in the stage-2 PTE. The write never passed through
> the host stage-1, and no unmap path reads the bit, so the record dies
> with the PTE and reclaim may discard written guest data (silent
> corruption).

Sorry, I didn't get what you are trying to achieve here.

On your patch, I see you marking backing pages dirty at wrprotect and unmap 
walkers, and it's very confusing to me on why you are doing this.

IIUC, what you are trying to do is make sure a page does not lose it's 
contents in case it is reclaimed by the OS.

IIUC, when a page is reclaimed, it will unmap the S2 PTE, and save it's 
contents to swap, so when the guest read or write to that page will cause 
it to fault. 

The page can never be just unmaped, as if there was a single write to it, 
the backing pages should be writable, and the contents need to be saved 
somewhere. 

So, if I was able to understand your suggestion, we don't need to worry 
about this, and this patch wouldn't be necessary. 

Did you hit this issue? 
Can you please expand on your issue, if I could not describe it above?

Thanks!
Leo