| Message ID | 6a7391fc-4abf-4e65-8159-30f4eae8fec5@suse.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | x86/shadow: replace p2m_is_valid() uses | expand |
On Wed, Mar 12, 2025 at 12:30:40PM +0100, Jan Beulich wrote: > The justification for dropping p2m_mmio_dm from p2m_is_valid() was wrong > for two of the shadow mode uses. > > In _sh_propagate() we want to create special L1 entries for p2m_mmio_dm > pages. Hence we need to make sure we don't bail early for that type. > > In _sh_page_fault() we want to handle p2m_mmio_dm by forwarding to > (internal or external) emulation. Pull the !p2m_is_mmio() check out of > the || expression (as otherwise it would need adding to the lhs as > well). > > In both cases, p2m_is_valid() in combination with p2m_is_grant() still > doesn't cover foreign mappings. Hence use p2m_is_any_ram() plus (as > necessary) p2m_mmio_* instead. > > Fixes: be59cceb2dbb ("x86/P2M: don't include MMIO_DM in p2m_is_valid()") > Reported-by: Luca Fancellu <Luca.Fancellu@arm.com> > Signed-off-by: Jan Beulich <jbeulich@suse.com> Acked-by: Roger Pau Monné <roger.pau@citrix.com> One suggestion below. > --- > This still leaves the p2m_mmio_dm vs p2m_invalid unaddressed. > > While propagating #PF to the guest based on P2M type isn't quite right, > not doing so in sh_page_fault() would lead to no forward progress in the > guest anymore: If we put in place a non-present shadow PTE, another #PF > will occur right after exiting to the guest. Doing so is only okay as a > transient measure, e.g. while paging back in a paged-out page (where the > respective type is included in P2M_RAM_TYPES). > > There could likely be at least one more Fixes: tag (to cover the lack of > handling foreign mappings); the one supplied is merely to indicate the > connection to the recent regression. > > --- a/xen/arch/x86/mm/shadow/multi.c > +++ b/xen/arch/x86/mm/shadow/multi.c > @@ -471,9 +471,7 @@ _sh_propagate(struct vcpu *v, > /* We don't shadow PAE l3s */ > ASSERT(GUEST_PAGING_LEVELS > 3 || level != 3); > > - /* Check there's something for the shadows to map to */ > - if ( (!p2m_is_valid(p2mt) && !p2m_is_grant(p2mt)) > - || !gfn_valid(d, target_gfn) ) > + if ( !gfn_valid(d, target_gfn) ) > { > *sp = shadow_l1e_empty(); > goto done; > @@ -503,6 +501,13 @@ _sh_propagate(struct vcpu *v, > goto done; > } > > + /* Check there's something for the shadows to map to */ > + if ( !p2m_is_any_ram(p2mt) && p2mt != p2m_mmio_direct ) > + { > + *sp = shadow_l1e_empty(); > + goto done; > + } > + > // Must have a valid target_mfn unless this is a prefetch or an l1 > // pointing at MMIO space. In the case of a prefetch, an invalid > // mfn means that we can not usefully shadow anything, and so we > @@ -2366,8 +2371,8 @@ static int cf_check sh_page_fault( > gmfn = get_gfn(d, gfn, &p2mt); > > if ( shadow_mode_refcounts(d) && > - ((!p2m_is_valid(p2mt) && !p2m_is_grant(p2mt)) || > - (!p2m_is_mmio(p2mt) && !mfn_valid(gmfn))) ) > + !p2m_is_mmio(p2mt) && > + (!p2m_is_any_ram(p2mt) || !mfn_valid(gmfn)) ) Would you mind adding some comment here about the need to forward p2m_mmio_dm to the emulation, and hence not possible to short-circuit here? Thanks, Roger.
Hi Jan, > On 12 Mar 2025, at 11:30, Jan Beulich <jbeulich@suse.com> wrote: > > The justification for dropping p2m_mmio_dm from p2m_is_valid() was wrong > for two of the shadow mode uses. > > In _sh_propagate() we want to create special L1 entries for p2m_mmio_dm > pages. Hence we need to make sure we don't bail early for that type. > > In _sh_page_fault() we want to handle p2m_mmio_dm by forwarding to > (internal or external) emulation. Pull the !p2m_is_mmio() check out of > the || expression (as otherwise it would need adding to the lhs as > well). > > In both cases, p2m_is_valid() in combination with p2m_is_grant() still > doesn't cover foreign mappings. Hence use p2m_is_any_ram() plus (as > necessary) p2m_mmio_* instead. > > Fixes: be59cceb2dbb ("x86/P2M: don't include MMIO_DM in p2m_is_valid()") > Reported-by: Luca Fancellu <Luca.Fancellu@arm.com> > Signed-off-by: Jan Beulich <jbeulich@suse.com> > Acked-by: Roger Pau Monné <roger.pau@citrix.com> > --- I’ve tested this one on top of the current staging and it fixes the XTF regression I was experiencing: Tested-by: Luca Fancellu <luca.fancellu@arm.com>
--- a/xen/arch/x86/mm/shadow/multi.c +++ b/xen/arch/x86/mm/shadow/multi.c @@ -471,9 +471,7 @@ _sh_propagate(struct vcpu *v, /* We don't shadow PAE l3s */ ASSERT(GUEST_PAGING_LEVELS > 3 || level != 3); - /* Check there's something for the shadows to map to */ - if ( (!p2m_is_valid(p2mt) && !p2m_is_grant(p2mt)) - || !gfn_valid(d, target_gfn) ) + if ( !gfn_valid(d, target_gfn) ) { *sp = shadow_l1e_empty(); goto done; @@ -503,6 +501,13 @@ _sh_propagate(struct vcpu *v, goto done; } + /* Check there's something for the shadows to map to */ + if ( !p2m_is_any_ram(p2mt) && p2mt != p2m_mmio_direct ) + { + *sp = shadow_l1e_empty(); + goto done; + } + // Must have a valid target_mfn unless this is a prefetch or an l1 // pointing at MMIO space. In the case of a prefetch, an invalid // mfn means that we can not usefully shadow anything, and so we @@ -2366,8 +2371,8 @@ static int cf_check sh_page_fault( gmfn = get_gfn(d, gfn, &p2mt); if ( shadow_mode_refcounts(d) && - ((!p2m_is_valid(p2mt) && !p2m_is_grant(p2mt)) || - (!p2m_is_mmio(p2mt) && !mfn_valid(gmfn))) ) + !p2m_is_mmio(p2mt) && + (!p2m_is_any_ram(p2mt) || !mfn_valid(gmfn)) ) { perfc_incr(shadow_fault_bail_bad_gfn); SHADOW_PRINTK("BAD gfn=%"SH_PRI_gfn" gmfn=%"PRI_mfn"\n",
The justification for dropping p2m_mmio_dm from p2m_is_valid() was wrong for two of the shadow mode uses. In _sh_propagate() we want to create special L1 entries for p2m_mmio_dm pages. Hence we need to make sure we don't bail early for that type. In _sh_page_fault() we want to handle p2m_mmio_dm by forwarding to (internal or external) emulation. Pull the !p2m_is_mmio() check out of the || expression (as otherwise it would need adding to the lhs as well). In both cases, p2m_is_valid() in combination with p2m_is_grant() still doesn't cover foreign mappings. Hence use p2m_is_any_ram() plus (as necessary) p2m_mmio_* instead. Fixes: be59cceb2dbb ("x86/P2M: don't include MMIO_DM in p2m_is_valid()") Reported-by: Luca Fancellu <Luca.Fancellu@arm.com> Signed-off-by: Jan Beulich <jbeulich@suse.com> --- This still leaves the p2m_mmio_dm vs p2m_invalid unaddressed. While propagating #PF to the guest based on P2M type isn't quite right, not doing so in sh_page_fault() would lead to no forward progress in the guest anymore: If we put in place a non-present shadow PTE, another #PF will occur right after exiting to the guest. Doing so is only okay as a transient measure, e.g. while paging back in a paged-out page (where the respective type is included in P2M_RAM_TYPES). There could likely be at least one more Fixes: tag (to cover the lack of handling foreign mappings); the one supplied is merely to indicate the connection to the recent regression.