Hi Stefano, > -----Original Message----- > From: Xen-devel <xen-devel-boun...@lists.xenproject.org> On Behalf Of > Stefano Stabellini > Sent: 2022年3月25日 9:01 > To: xen-devel@lists.xenproject.org > Cc: jul...@xen.org; sstabell...@kernel.org; Bertrand Marquis > <bertrand.marq...@arm.com>; volodymyr_babc...@epam.com; Stefano Stabellini > <stefano.stabell...@xilinx.com> > Subject: [PATCH] xen/arm: set CPSR Z bit when creating aarch32 guests > > From: Stefano Stabellini <stefano.stabell...@xilinx.com> > > The first 32 bytes of zImage are NOPs. When CONFIG_EFI is enabled in the > kernel, certain versions of Linux will use an UNPREDICATABLE NOP > encoding, sometimes resulting in an unbootable kernel. Whether the > resulting kernel is bootable or not depends on the processor. See commit > a92882a4d270 in the Linux kernel for all the details. > > All kernel releases starting from Linux 4.9 without commit a92882a4d270 > are affected. > > Fortunately there is a simple workaround: setting the "Z" bit in CPSR > make it so those invalid NOP instructions are never executed. That is > because the instruction is conditional (not equal). So, on QEMU at > least, the instruction will end up to be ignored and not generate an > exception. Setting the "Z" bit makes those kernel versions bootable > again and it is harmless in the other cases. > > Note that both U-Boot and QEMU -kernel set the "Z" bit in CPSR when > booting a zImage kernel on aarch32. > > Signed-off-by: Stefano Stabellini <stefano.stabell...@xilinx.com> > --- > Changes in v3: > - improve commit message > - improve in-code comment > > Changes in v2: > - improve commit message > - add in-code comment > - move PSR_Z to the beginning > --- > xen/include/public/arch-arm.h | 11 ++++++++++- > 1 file changed, 10 insertions(+), 1 deletion(-) > > diff --git a/xen/include/public/arch-arm.h b/xen/include/public/arch-arm.h > index 94b31511dd..c0c1149e27 100644 > --- a/xen/include/public/arch-arm.h > +++ b/xen/include/public/arch-arm.h > @@ -361,6 +361,7 @@ typedef uint64_t xen_callback_t; > #define PSR_DBG_MASK (1<<9) /* arm64: Debug Exception mask */ > #define PSR_IT_MASK (0x0600fc00) /* Thumb If-Then Mask */ > #define PSR_JAZELLE (1<<24) /* Jazelle Mode */ > +#define PSR_Z (1<<30) /* Zero condition flag */ > > /* 32 bit modes */ > #define PSR_MODE_USR 0x10 > @@ -383,7 +384,15 @@ typedef uint64_t xen_callback_t; > #define PSR_MODE_EL1t 0x04 > #define PSR_MODE_EL0t 0x00 > > -#define PSR_GUEST32_INIT > (PSR_ABT_MASK|PSR_FIQ_MASK|PSR_IRQ_MASK|PSR_MODE_SVC) > +/* > + * We set PSR_Z to be able to boot Linux kernel versions with an invalid > + * encoding of the first 8 NOP instructions. See commit a92882a4d270 in > + * Linux. > + * > + * Note that PSR_Z is also set by U-Boot and QEMU -kernel when loading > + * zImage kernels on aarch32. > + */ > +#define PSR_GUEST32_INIT > (PSR_Z|PSR_ABT_MASK|PSR_FIQ_MASK|PSR_IRQ_MASK|PSR_MODE_SVC) > #define PSR_GUEST64_INIT > (PSR_ABT_MASK|PSR_FIQ_MASK|PSR_IRQ_MASK|PSR_MODE_EL1h) >
Maybe this is a good opportunity to fix the alignment of the two macros : ) Reviewed-by: Wei Chen <wei.c...@arm.com> > #define SCTLR_GUEST_INIT xen_mk_ullong(0x00c50078) > -- > 2.25.1 >
