linux-kernel - RE: [PATCH v3 1/3] x86/hyperv: Use slow_virt_to_phys() in page transition hypervisor callback

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Date: Mon, 8 Jan 2024 16:00:34 +0000
From: Michael Kelley <mhklinux@...look.com>
To: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
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Subject: RE: [PATCH v3 1/3] x86/hyperv: Use slow_virt_to_phys() in page
 transition hypervisor callback

From: kirill.shutemov@...ux.intel.com <kirill.shutemov@...ux.intel.com> Sent: Monday, January 8, 2024 5:08 AM
> 
> On Fri, Jan 05, 2024 at 10:30:23AM -0800, mhkelley58@...il.com wrote:
> > From: Michael Kelley <mhklinux@...look.com>
> >
> > In preparation for temporarily marking pages not present during a
> > transition between encrypted and decrypted, use slow_virt_to_phys()
> > in the hypervisor callback. As long as the PFN is correct,
> > slow_virt_to_phys() works even if the leaf PTE is not present.
> > The existing functions that depend on vmalloc_to_page() all
> > require that the leaf PTE be marked present, so they don't work.
> >
> > Update the comments for slow_virt_to_phys() to note this broader usage
> > and the requirement to work even if the PTE is not marked present.
> >
> > Signed-off-by: Michael Kelley <mhklinux@...look.com>
> > ---
> >  arch/x86/hyperv/ivm.c        |  9 ++++++++-
> >  arch/x86/mm/pat/set_memory.c | 13 +++++++++----
> >  2 files changed, 17 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/hyperv/ivm.c b/arch/x86/hyperv/ivm.c
> > index 02e55237d919..8ba18635e338 100644
> > --- a/arch/x86/hyperv/ivm.c
> > +++ b/arch/x86/hyperv/ivm.c
> > @@ -524,7 +524,14 @@ static bool hv_vtom_set_host_visibility(unsigned
> long kbuffer, int pagecount, bo
> >  		return false;
> >
> >  	for (i = 0, pfn = 0; i < pagecount; i++) {
> > -		pfn_array[pfn] = virt_to_hvpfn((void *)kbuffer + i * HV_HYP_PAGE_SIZE);
> > +		/*
> > +		 * Use slow_virt_to_phys() because the PRESENT bit has been
> > +		 * temporarily cleared in the PTEs.  slow_virt_to_phys() works
> > +		 * without the PRESENT bit while virt_to_hvpfn() or similar
> > +		 * does not.
> > +		 */
> > +		pfn_array[pfn] = slow_virt_to_phys((void *)kbuffer +
> > +					i * HV_HYP_PAGE_SIZE) >> HV_HYP_PAGE_SHIFT;
> 
> I think you can make it much more readable by introducing few variables:
> 
> 		virt = (void *)kbuffer + i * HV_HYPPAGE_SIZE;
> 		phys = slow_virt_to_phys(virt);
> 		pfn_array[pfn] = phys >> HV_HYP_PAGE_SHIFT;
> 

Agreed.  I'll do this in the next version.

Michael