[Xen-devel] [PATCH] x86/boot: re-arrange how/when we do disk I/O

["Jan Beulich" <JBeulich@xxxxxxxx>]

We place the trampoline no lower than at 256k, so we have ample space
to read the MBRs of BIOS disks into an aligned buffer right below the
trampoline (not doing so has been found to be a problem on a buggy BIOS
coming with a Skull Canyon NUC). To facilitate that move MBR reading
past EDD info retrieval.

Also add a wrap check to the EDD info retrieval loop, to match that in
the MBR reading one.

Reported-by: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Tested-by: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
---
Using 512-byte sector size as default right now - perhaps worth
considering to use 4k instead. I'm also not sure whether we shouldn't
sanity check the sector size some more.

--- a/xen/arch/x86/boot/edd.S
+++ b/xen/arch/x86/boot/edd.S
@@ -26,46 +26,6 @@
 get_edd:
         cmpb    $2, bootsym(opt_edd)            # edd=off ?
         je      edd_done
-        cmpb    $1, bootsym(opt_edd)            # edd=skipmbr ?
-        je      edd_start
-
-# Read the first sector of each BIOS disk device and store the 4-byte signature
-edd_mbr_sig_start:
-        movb    $0x80, %dl                      # from device 80
-        movw    $bootsym(boot_mbr_signature),%bx # store buffer ptr in bx
-edd_mbr_sig_read:
-        pushw   %bx
-        movb    $0x02, %ah                      # 0x02 Read Sectors
-        movb    $1, %al                         # read 1 sector
-        movb    $0, %dh                         # at head 0
-        movw    $1, %cx                         # cylinder 0, sector 0
-        pushw   %es
-        pushw   %ds
-        popw    %es
-        movw    $bootsym(boot_edd_info), %bx    # disk's data goes into info
-        pushw   %dx             # work around buggy BIOSes
-        stc                     # work around buggy BIOSes
-        int     $0x13
-        sti                     # work around buggy BIOSes
-        popw    %dx
-        popw    %es
-        popw    %bx
-        jc      edd_mbr_sig_done                # on failure, we're done.
-        cmpb    $0, %ah                         # some BIOSes do not set CF
-        jne     edd_mbr_sig_done                # on failure, we're done.
-        cmpw    $0xaa55, bootsym(boot_edd_info)+0x1fe
-        jne     .Ledd_mbr_sig_next
-        movl    bootsym(boot_edd_info)+EDD_MBR_SIG_OFFSET,%eax
-        movb    %dl, (%bx)                      # store BIOS drive number
-        movl    %eax, 4(%bx)                    # store signature from MBR
-        incb    bootsym(boot_mbr_signature_nr)  # note that we stored something
-        addw    $8, %bx                         # increment sig buffer ptr
-.Ledd_mbr_sig_next:
-        incb    %dl                             # increment to next device
-        jz      edd_mbr_sig_done
-        cmpb    $EDD_MBR_SIG_MAX,bootsym(boot_mbr_signature_nr)
-        jb      edd_mbr_sig_read
-edd_mbr_sig_done:
 
 # Do the BIOS Enhanced Disk Drive calls
 # This consists of two calls:
@@ -136,10 +96,71 @@ edd_legacy_done:
 
 edd_next:
         incb    %dl                             # increment to next device
+        jz      edd_done
         cmpb    $EDD_INFO_MAX,bootsym(boot_edd_info_nr)
         jb      edd_check_ext
 
 edd_done:
+        cmpb    $1, bootsym(opt_edd)            # edd=skipmbr ?
+        je      .Ledd_mbr_sig_skip
+
+# Read the first sector of each BIOS disk device and store the 4-byte signature
+.Ledd_mbr_sig_start:
+        pushw   %es
+        movb    $0x80, %dl                      # from device 80
+        movw    $bootsym(boot_mbr_signature), %bx # store buffer ptr in bx
+.Ledd_mbr_sig_read:
+        pushw   %bx
+        movw    $bootsym(boot_edd_info), %bx
+        movzbw  bootsym(boot_edd_info_nr), %cx
+        jcxz    .Ledd_mbr_sig_default
+.Ledd_mbr_sig_find_info:
+        cmpb    %dl, (%bx)
+        ja      .Ledd_mbr_sig_default
+        je      .Ledd_mbr_sig_get_size
+        add     $EDDEXTSIZE+EDDPARMSIZE, %bx
+        loop    .Ledd_mbr_sig_find_info
+.Ledd_mbr_sig_default:
+        movw    $(512 >> 4), %bx
+        jmp     .Ledd_mbr_sig_set_buf
+.Ledd_mbr_sig_get_size:
+        movw    EDDEXTSIZE+0x18(%bx), %bx       # sector size
+        shr     $4, %bx                         # convert to paragraphs
+        jz      .Ledd_mbr_sig_default
+.Ledd_mbr_sig_set_buf:
+        movw    %ds, %ax
+        subw    %bx, %ax                        # disk's data goes right ahead
+        movw    %ax, %es                        # of trampoline
+        xorw    %bx, %bx
+        movw    %bx, %es:0x1fe(%bx)             # clear BIOS magic just in case
+        pushw   %dx                             # work around buggy BIOSes
+        stc                                     # work around buggy BIOSes
+        movw    $0x0201, %ax                    # read 1 sector
+        movb    $0, %dh                         # at head 0
+        movw    $1, %cx                         # cylinder 0, sector 0
+        int     $0x13
+        sti                                     # work around buggy BIOSes
+        popw    %dx
+        movw    %es:0x1fe(%bx), %si
+        movl    %es:EDD_MBR_SIG_OFFSET(%bx), %ecx
+        popw    %bx
+        jc      .Ledd_mbr_sig_done              # on failure, we're done.
+        testb   %ah, %ah                        # some BIOSes do not set CF
+        jnz     .Ledd_mbr_sig_done              # on failure, we're done.
+        cmpw    $0xaa55, %si
+        jne     .Ledd_mbr_sig_next
+        movb    %dl, (%bx)                      # store BIOS drive number
+        movl    %ecx, 4(%bx)                    # store signature from MBR
+        incb    bootsym(boot_mbr_signature_nr)  # note that we stored something
+        addw    $8, %bx                         # increment sig buffer ptr
+.Ledd_mbr_sig_next:
+        incb    %dl                             # increment to next device
+        jz      .Ledd_mbr_sig_done
+        cmpb    $EDD_MBR_SIG_MAX, bootsym(boot_mbr_signature_nr)
+        jb      .Ledd_mbr_sig_read
+.Ledd_mbr_sig_done:
+        popw    %es
+.Ledd_mbr_sig_skip:
         ret
 
 GLOBAL(boot_edd_info_nr)
@@ -149,4 +170,4 @@ GLOBAL(boot_mbr_signature_nr)
 GLOBAL(boot_mbr_signature)
         .fill   EDD_MBR_SIG_MAX*8,1,0
 GLOBAL(boot_edd_info)
-        .fill   512,1,0                         # big enough for a disc sector
+        .fill   EDD_INFO_MAX * (EDDEXTSIZE + EDDPARMSIZE), 1, 0

Attachment: x86-MBR-below-trampoline.patch
Description: Text document

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