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Tue, 31 Mar 2026 01:21:47 -0700 (PDT) From: Andrew Cooper To: xen-devel@lists.xenproject.org Cc: Andrew Cooper , Jan Beulich , =?UTF-8?q?Roger=20Pau=20Monn=C3=A9?= Subject: [PATCH v2] x86/pv: Provide better SYSCALL backwards compatibility in FRED mode Date: Tue, 31 Mar 2026 09:21:42 +0100 Message-Id: <20260331082142.13254-1-andrew.cooper3@citrix.com> X-Mailer: git-send-email 2.34.1 MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-purgate-ID: tlsNG-ef75cf/1774945308-55EB3303-DA90FA0A/0/0 X-purgate-type: clean X-purgate-size: 3856 X-ZohoMail-DKIM: pass (identity @citrix.com) X-ZM-MESSAGEID: 1774945325893158500 In FRED mode, the SYSCALL instruction does not modify %rcx/%r11. Software using SYSCALL spills %rcx/%r11 around the invocation, which is why FRED not doing this goes largely unnoticed. Nevertheless, there is a difference in the guest immediately following cases where Xen would use SYSRET in non-FRED mode. These are: * HYPERCALL_iret with VGCF_in_syscall set. * Delivery to a registered entrypoint. In the PV ABI this is always SYSRE= T'd to, with %rcx/%r11 provided on the stack. Debugging or snapshotting activities will observe the stale contents of %rcx/%r11 in FRED mode, rather than the %rip/eflags value they'd have in IDT mode. Manually adjust them when SYSRET would have been used. Regarding the choice of instructions in eretu_exit_to_guest(), a branch wou= ld be a context dependent 50/50 split (i.e. increased chance of mispredict), a= nd only saves one instruction. The CMOVs read the same cacheline that ERETU is about to process, so are as close to free as we can reasonably get. Fixes: 76193ef47d91 ("x86/pv: System call handling in FRED mode") Signed-off-by: Andrew Cooper Reviewed-by: Jan Beulich --- CC: Jan Beulich CC: Roger Pau Monn=C3=A9 v2: * Rewrite the commit message. * Set TRAP_syscall in the SYSCALL path. --- xen/arch/x86/traps.c | 9 +++++---- xen/arch/x86/x86_64/entry-fred.S | 12 +++++++++++- 2 files changed, 16 insertions(+), 5 deletions(-) diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c index 8aa1e4181bd1..656ad337ab90 100644 --- a/xen/arch/x86/traps.c +++ b/xen/arch/x86/traps.c @@ -2392,10 +2392,6 @@ void asmlinkage entry_from_pv(struct cpu_user_regs *= regs) * The guest isn't aware of FRED, so recreate the legacy * behaviour. * - * The non-FRED SYSCALL path sets TRAP_syscall in entry_vector= to - * signal that SYSRET can be used, but this isn't relevant in = FRED - * mode. - * * When setting the selectors, clear all upper metadata again = for * backwards compatibility. In particular fred_ss.swint becom= es * pend_DB on ERETx, and nothing else in the pv_hypercall() wo= uld @@ -2411,9 +2407,14 @@ void asmlinkage entry_from_pv(struct cpu_user_regs *= regs) =20 regs->ssx =3D l ? FLAT_KERNEL_SS : FLAT_USER_SS32; regs->csx =3D l ? FLAT_KERNEL_CS64 : FLAT_USER_CS32; + regs->rcx =3D regs->rip; + regs->r11 =3D regs->rflags; =20 if ( guest_kernel_mode(curr, regs) ) + { + regs->entry_vector |=3D TRAP_syscall; pv_hypercall(regs); + } else if ( (l ? curr->arch.pv.syscall_callback_eip : curr->arch.pv.syscall32_callback_eip) =3D=3D 0 ) { diff --git a/xen/arch/x86/x86_64/entry-fred.S b/xen/arch/x86/x86_64/entry-f= red.S index 2fa57beb930c..e9c84423dacd 100644 --- a/xen/arch/x86/x86_64/entry-fred.S +++ b/xen/arch/x86/x86_64/entry-fred.S @@ -4,6 +4,7 @@ =20 #include #include +#include =20 .section .text.entry, "ax", @progbits =20 @@ -26,7 +27,16 @@ FUNC(entry_FRED_R3, 4096) END(entry_FRED_R3) =20 FUNC(eretu_exit_to_guest) - POP_GPRS + /* + * PV guests aren't aware of FRED. If Xen in IDT mode would have = used + * a SYSRET instruction, preserve the legacy behaviour for %rcx/%r= 11 + */ + testb $TRAP_syscall >> 8, UREGS_entry_vector + 1(%rsp) + + POP_GPRS /* Preserves flags */ + + cmovnz EFRAME_rip(%rsp), %rcx + cmovnz EFRAME_eflags(%rsp), %r11 =20 /* * Exceptions here are handled by redirecting either to --=20 2.34.1