On Thu, May 01, 2025, H. Peter Anvin wrote: > On May 1, 2025 11:30:18 AM PDT, Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > >On Thu, May 01, 2025, Peter Zijlstra wrote: > >> On Thu, May 01, 2025 at 12:30:38PM +0200, Peter Zijlstra wrote: > >> > On Wed, Apr 30, 2025 at 09:06:00PM +0200, Peter Zijlstra wrote: > >> > > On Wed, Apr 30, 2025 at 07:24:15AM -0700, H. Peter Anvin wrote: > >> > > > >> > > > >KVM has another; the VMX interrupt injection stuff calls the IDT handler > >> > > > >directly. Is there an alternative? Can we keep a table of Linux functions > >> > > > >slighly higher up the call stack (asm_\cfunc ?) and add CFI to those? > >> > > > >> > > > We do have a table of handlers higher up in the stack in the form of > >> > > > the dispatch tables for FRED. They don't in general even need the > >> > > > assembly entry stubs, either. > >> > > > >> > > Oh, right. I'll go have a look at those. > >> > > >> > Right, so perhaps the easiest way around this is to setup the FRED entry > >> > tables unconditionally, have VMX mandate CONFIG_FRED and then have it > >> > always use the FRED entry points. > >> > > >> > Let me see how ugly that gets. > >> > >> Something like so... except this is broken. Its reporting spurious > >> interrupts on vector 0x00, so something is buggered passing that vector > >> along. > > > >Uh, aren't you making this way more complex than it needs to be? IIUC, KVM never > >uses the FRED hardware entry points, i.e. the FRED entry tables don't need to be > >in place because they'll never be used. The only bits of code KVM needs is the > >__fred_entry_from_kvm() glue. > > > >Lightly tested, but this combo works for IRQs and NMIs on non-FRED hardware. Hrm, and now I see that fred_extint() relies on fred_install_sysvec(), which makes me quite curious as to why IRQs didn't go sideways. Oh, because sysvec_table[] is statically defined at compile time except for PV crud. So yeah, I think my the patches are correct, they just the need a small bit of prep work to support dynamic setup of sysvec_table. > >-- > >From 664468143109ab7c525c0babeba62195fa4c657e Mon Sep 17 00:00:00 2001 > >From: Sean Christopherson <seanjc@xxxxxxxxxx> > >Date: Thu, 1 May 2025 11:20:29 -0700 > >Subject: [PATCH 1/2] x86/fred: Play nice with invoking > > asm_fred_entry_from_kvm() on non-FRED hardware > > > >Modify asm_fred_entry_from_kvm() to allow it to be invoked by KVM even > >when FRED isn't fully enabled, e.g. when running with CONFIG_X86_FRED=y > >on non-FRED hardware. This will allow forcing KVM to always use the FRED > >entry points for 64-bit kernels, which in turn will eliminate a rather > >gross non-CFI indirect call that KVM uses to trampoline IRQs by doing IDT > >lookups. > > > >When FRED isn't enabled, simply skip ERETS and restore RBP and RSP from > >the stack frame prior to doing a "regular" RET back to KVM (in quotes > >because of all the RET mitigation horrors). > > > >Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> > >--- > > arch/x86/entry/entry_64_fred.S | 5 +++-- > > 1 file changed, 3 insertions(+), 2 deletions(-) > > > >diff --git a/arch/x86/entry/entry_64_fred.S b/arch/x86/entry/entry_64_fred.S > >index 29c5c32c16c3..7aff2f0a285f 100644 > >--- a/arch/x86/entry/entry_64_fred.S > >+++ b/arch/x86/entry/entry_64_fred.S > >@@ -116,7 +116,8 @@ SYM_FUNC_START(asm_fred_entry_from_kvm) > > movq %rsp, %rdi /* %rdi -> pt_regs */ > > call __fred_entry_from_kvm /* Call the C entry point */ > > POP_REGS > >- ERETS > >+ > >+ ALTERNATIVE "", __stringify(ERETS), X86_FEATURE_FRED > > 1: > > /* > > * Objtool doesn't understand what ERETS does, this hint tells it that > >@@ -124,7 +125,7 @@ SYM_FUNC_START(asm_fred_entry_from_kvm) > > * isn't strictly needed, but it's the simplest form. > > */ > > UNWIND_HINT_RESTORE > >- pop %rbp > >+ leave > > RET > > > > SYM_FUNC_END(asm_fred_entry_from_kvm) > > > >base-commit: 45eb29140e68ffe8e93a5471006858a018480a45 > > Ok maybe I'm being dense, but what is left other than simply calling > __fred_entry_from_kvm() as a normal C function? > > I'm on the go so there might be something in the code I'm missing, but on the > surface...? I'm sure it's doable, though I'd be more than a little nervous about diverging from what FRED=y does, e.g. in case code somewhere expects the stack to look exactly like a real FRED event. And since we'd still need the assembly to support FRED=y, I don't see any point in adding more code when it's trivially easy to have asm_fred_entry_from_kvm() skip ERETS.
