linux-kernel - Re: [PATCH v2 2/4] mm: x86: Invoke hypercall when page encryption status is changed

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Date:   Thu, 13 May 2021 04:34:41 +0000
From:   Ashish Kalra <ashish.kalra@....com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     seanjc@...gle.com, pbonzini@...hat.com, tglx@...utronix.de,
        mingo@...hat.com, hpa@...or.com, joro@...tes.org,
        thomas.lendacky@....com, x86@...nel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, srutherford@...gle.com,
        venu.busireddy@...cle.com, brijesh.singh@....com
Subject: Re: [PATCH v2 2/4] mm: x86: Invoke hypercall when page encryption
 status is changed

Hello Boris,

On Wed, May 12, 2021 at 03:15:37PM +0200, Borislav Petkov wrote:
> On Fri, Apr 23, 2021 at 03:58:43PM +0000, Ashish Kalra wrote:
> > +static inline void notify_page_enc_status_changed(unsigned long pfn,
> > +						  int npages, bool enc)
> > +{
> > +	PVOP_VCALL3(mmu.notify_page_enc_status_changed, pfn, npages, enc);
> > +}
> 
> Now the question is whether something like that is needed for TDX, and,
> if so, could it be shared by both.
> 
> Sean?
> 
> > +void notify_addr_enc_status_changed(unsigned long vaddr, int npages,
> > +				    bool enc)
> 
> Let that line stick out.
> 
> > +{
> > +#ifdef CONFIG_PARAVIRT
> > +	unsigned long sz = npages << PAGE_SHIFT;
> > +	unsigned long vaddr_end = vaddr + sz;
> > +
> > +	while (vaddr < vaddr_end) {
> > +		int psize, pmask, level;
> > +		unsigned long pfn;
> > +		pte_t *kpte;
> > +
> > +		kpte = lookup_address(vaddr, &level);
> > +		if (!kpte || pte_none(*kpte))
> > +			return;
> 
> What does this mean exactly? On the first failure to lookup the address,
> you return? Why not continue so that you can notify about the remaining
> pages in [vaddr - vaddr_end)?

What's the use of notification of a partial page list, even a single
incorrect guest page encryption status can crash the guest/migrated
guest.

> Also, what does it mean for the current range if the lookup fails?
> Innocuous situation or do you need to signal it with a WARN or so?
> 

Yes, it makes sense to signal it with a WARN or so.

> > +
> > +		pfn = pg_level_to_pfn(level, kpte, NULL);
> > +		if (!pfn)
> > +			continue;
> 
> Same here: if it hits the default case, wouldn't it make sense to
> WARN_ONCE or so to catch potential misuse? Or better yet, the WARN_ONCE
> should be in pg_level_to_pfn().

Yes, it makes sense to add a WARN_ONCE() in pg_level_to_pfn().
> 
> > diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> > index 16f878c26667..45e65517405a 100644
> > --- a/arch/x86/mm/pat/set_memory.c
> > +++ b/arch/x86/mm/pat/set_memory.c
> > @@ -2012,6 +2012,13 @@ static int __set_memory_enc_dec(unsigned long addr, int numpages, bool enc)
> >  	 */
> >  	cpa_flush(&cpa, 0);
> >  
> > +	/*
> > +	 * Notify hypervisor that a given memory range is mapped encrypted
> > +	 * or decrypted. The hypervisor will use this information during the
> > +	 * VM migration.
> > +	 */
> > +	notify_addr_enc_status_changed(addr, numpages, enc);
> 
> If you notify about a range then that function should be called
> 
> 	notify_range_enc_status_changed
> 

Ok. 

Thanks,
Ashish

> or so.
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
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