fs/v9fs: Split virtio buffers where their physical pages are not adjacent.

[casaroli]

Summary

virtio_9p_request() gave each buffer to the device as one descriptor, and libmetal translates only its start address with up_addrenv_va_to_pa(). In a kernel build, a buffer in a user address environment is contiguous only in virtual memory. A read that crossed a page boundary was written into whatever physical page followed the first one: the data was wrong and other memory was overwritten. When that range left guest memory, QEMU stopped the device with "virtio: bogus descriptor or out of resources".

With CONFIG_ARCH_ADDRENV, this PR splits each buffer at the pages that are not physically adjacent, so each descriptor covers one physical range.

Impact

Builds with CONFIG_ARCH_ADDRENV and the virtio 9P transport. Other builds are unchanged.

Testing

qemu-armv8a:knsh with CONFIG_DRIVERS_VIRTIO, CONFIG_DRIVERS_VIRTIO_MMIO, CONFIG_FS_V9FS and CONFIG_V9FS_VIRTIO_9P, QEMU 11 (-m 512M, a virtio-9p-device share). The test programs were built outside the tree against the export package.

A program reads a 1 MB file from the 9P share in one read() into a heap buffer and checks every byte. Two other programs run first, so that the heap pages of the test program are not physically contiguous: hold 512 3 & (exits after 3 s) and hold 64 120 &.

Before:

nsh> readbig /mnt/data.bin
readbig: read 1048576 bytes, first wrong byte at 462840

After:

nsh> readbig /mnt/data.bin
readbig: read 1048576 bytes, all correct

[github-actions]

MemBrowse Memory Report

qemu-armv8a

• Code: .text.init_builtin_run +60 B (+0.0%, 347,480 B)

qemu-intel64

• Code: .text +74 B (+0.0%, 8,682,514 B)
  No memory changes detected for:
• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb