Re: [PATCH v2 4/4] i386/hvf: Fixes dirty memory tracking by page granularity RX->RWX change

[Roman Bolshakov]

On Sat, Oct 21, 2023 at 10:05:18PM +0200, Phil Dennis-Jordan wrote:
> When using x86 macOS Hypervisor.framework as accelerator, detection of
> dirty memory regions is implemented by marking logged memory region
> slots as read-only in the EPT, then setting the dirty flag when a
> guest write causes a fault. The area marked dirty should then be marked
> writable in order for subsequent writes to succeed without a VM exit.
> 
> However, dirty bits are tracked on a per-page basis, whereas the fault
> handler was marking the whole logged memory region as writable. This
> change fixes the fault handler so only the protection of the single
> faulting page is marked as dirty.
> 
> (Note: the dirty page tracking appeared to work despite this error
> because HVF’s hv_vcpu_run() function generated unnecessary EPT fault
> exits, which ended up causing the dirty marking handler to run even
> when the memory region had been marked RW. When using
> hv_vcpu_run_until(), a change planned for a subsequent commit, these
> spurious exits no longer occur, so dirty memory tracking malfunctions.)
> 
> Additionally, the dirty page is set to permit code execution, the same
> as all other guest memory; changing memory protection from RX to RW not
> RWX appears to have been an oversight.
> 

Hi Phil, I don't observe a problem with SVGA if I apply CPU kick patch
on top of it. Thanks for fixing this,

Reviewed-by: Roman Bolshakov <roman@roolebo.dev>
Tested-by: Roman Bolshakov <roman@roolebo.dev>

Regards,
Roman