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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH early-RFC 4/5] xen/arm: mm: Rework switch_ttbr()
On Sat, 12 Mar 2022, Julien Grall wrote:
> On 12/03/2022 01:31, Stefano Stabellini wrote:
> > On Wed, 9 Mar 2022, Julien Grall wrote:
> > > From: Julien Grall <jgrall@xxxxxxxxxx>
> > >
> > > At the moment, switch_ttbr() is switching the TTBR whilst the MMU is
> > > still on.
> > >
> > > Switching TTBR is like replacing existing mappings with new ones. So
> > > we need to follow the break-before-make sequence.
> > >
> > > In this case, it means the MMU needs to be switched off while the
> > > TTBR is updated. In order to disable the MMU, we need to first
> > > jump to an identity mapping.
> > >
> > > Rename switch_ttbr() to switch_ttbr_id() and create an helper on
> > > top to temporary map the identity mapping and call switch_ttbr()
> > > via the identity address.
> > >
> > > switch_ttbr_id() is now reworked to temporarily turn off the MMU
> > > before updating the TTBR.
> > >
> > > We also need to make sure the helper switch_ttbr() is part of the
> > > identity mapping. So move _end_boot past it.
> > >
> > > Take the opportunity to instruction cache flush as the operation is
> > > only necessary when the memory is updated.
> > >
> > > Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
> > >
> > > ---
> > >
> > > TODO:
> > > * Rename _end_boot to _end_id_mapping or similar
> > > * Check the memory barriers
> > > * I suspect the instruction cache flush will be necessary
> > > for cache coloring.
> > > ---
> > > xen/arch/arm/arm64/head.S | 31 ++++++++++++++++++++-----------
> > > xen/arch/arm/mm.c | 14 +++++++++++++-
> > > 2 files changed, 33 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
> > > index 878649280d73..c5cc72b8fe6f 100644
> > > --- a/xen/arch/arm/arm64/head.S
> > > +++ b/xen/arch/arm/arm64/head.S
> > > @@ -803,36 +803,45 @@ fail: PRINT("- Boot failed -\r\n")
> > > b 1b
> > > ENDPROC(fail)
> > > -GLOBAL(_end_boot)
> > > -
> > > /*
> > > * Switch TTBR
> > > *
> > > * x0 ttbr
> > > *
> > > - * TODO: This code does not comply with break-before-make.
> > > + * XXX: Check the barriers
> > > */
> > > -ENTRY(switch_ttbr)
> > > +ENTRY(switch_ttbr_id)
> > > dsb sy /* Ensure the flushes happen before
> > > * continuing */
> > > isb /* Ensure synchronization with
> > > previous
> > > * changes to text */
> > > +
> > > + /* Turn off MMU */
> > > + mrs x1, SCTLR_EL2
> > > + bic x1, x1, #SCTLR_Axx_ELx_M
> > > + msr SCTLR_EL2, x1
> > > + dsb sy
> > > + isb
> > > +
> > > tlbi alle2 /* Flush hypervisor TLB */
> > > - ic iallu /* Flush I-cache */
> > > dsb sy /* Ensure completion of TLB flush
> > > */
> > > isb
> > > - msr TTBR0_EL2, x0
> > > + msr TTBR0_EL2, x0
> > > +
> > > + mrs x1, SCTLR_EL2
> > > + orr x1, x1, #SCTLR_Axx_ELx_M /* Enable MMU */
> > > + msr SCTLR_EL2, x1
> > > isb /* Ensure synchronization with
> > > previous
> > > * changes to text */
> > > - tlbi alle2 /* Flush hypervisor TLB */
> > > - ic iallu /* Flush I-cache */
> > > - dsb sy /* Ensure completion of TLB flush */
> > > - isb
> > > + /* Turn on the MMU */
> > > +
> > > ret
> > > -ENDPROC(switch_ttbr)
> > > +ENDPROC(switch_ttbr_id)
> > > +
> > > +GLOBAL(_end_boot)
> > > #ifdef CONFIG_EARLY_PRINTK
> > > /*
> > > diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> > > index 5c4dece16f7f..a53760af7af0 100644
> > > --- a/xen/arch/arm/mm.c
> > > +++ b/xen/arch/arm/mm.c
> > > @@ -660,7 +660,19 @@ static void xen_pt_enforce_wnx(void)
> > > flush_xen_tlb_local();
> > > }
> > > -extern void switch_ttbr(uint64_t ttbr);
> > > +extern void switch_ttbr_id(uint64_t ttbr);
> > > +
> > > +typedef void (switch_ttbr_fn)(uint64_t ttbr);
> > > +
> > > +static void switch_ttbr(uint64_t ttbr)
> > > +{
> > > + vaddr_t id_addr = virt_to_maddr(switch_ttbr_id);
> > > + switch_ttbr_fn *fn = (switch_ttbr_fn *)id_addr;
> > > +
> > > + update_identity_mapping(true);
> > > + fn(ttbr);
> > > + update_identity_mapping(false);
> > > +}
> >
> > Controversial question: does it really matter that XEN_VIRT_START >
> > 512GB and that _start < 512GB?
> >
> > I am totally fine with the limit, I am just brainstorming: given that
> > the mapping is used very temporarely, it wouldn't really be an issue if
> > it conflicts with something important. Let's say that it conflicts with
> > the VMAP or the FRAMETABLE. As long as:
> >
> > - we save the current mapping
> > - update it with the Xen 1:1
> > - switch_ttbr
> > - remove Xen 1:1
> > - restore mapping
> >
> > It should work, right? Basically, a mapping conflict shouldn't be an
> > issue given that the mapping has only to live long enough to call
> > switch_ttbr_id.
>
> Today switch_ttbr() is called before we initialized most of the memory layout.
> So clashing with the VMAP and frametable is not a problem.
>
> However, the identity mapping may also clash with the region used to map Xen.
> That said, technically, we are not able to handle Xen when its start address
> is in region 2MB + 4K to 4MB (Xen is loaded at a 4KB aligned address).
>
> The trouble is some features (e.g. UBSAN, GCOV) can generate Xen image over
> 2MB. IOW, the range where Xen cannot be loaded will increase.
>
> This is an issue because AFAIK, there is no away to tell GRUB "You can't load
> Xen at this region". But even if there were one, I feel this restriction is
> sort of random.
>
> I already wrote a patch to get rid of the restriction. The code is not too bad
> (we only need an extra indirection). But I haven't sent it yet because it is
> less critical with the re-shuffling of the memory layout.
Interesting! I am curious: how did you manage to do it?
For now and for this series the current approach and the 512GB limit are
fine. My replies here are brainstorming to see if there are potential
alternatives in the future in case the need arises.
I can see that a clash with Xen mapping could be problematic and the
chances of that happening are low but non-zero. We could make sure that
ImageBuilder always picks safe addresses and that would help but
wouldn't remove the issue if someone is not using ImageBuilder.
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