test(virtio-net): have a CI test with small receive buffers

In order to ensure that the buffer merging works, make the receive
buffers smaller in a CI step. Note, however, that this does not check if
the MRG_RXBUF is available for the step. If it's not, the environment
variable will have no effect.

Author: cagatay-y

.github/workflows/ci.yml

@@ -235,6 +235,13 @@ jobs:
         working-directory: .
       - run: cargo xtask ci rs --arch ${{ matrix.arch }} --profile ${{ matrix.profile }} --package httpd --features ci,hermit/dhcpv4,hermit/virtio-net qemu ${{ matrix.flags }} --devices virtio-net-pci
         if: matrix.arch != 'riscv64'
+      # FIXME: this is broken on QEMU 8.2.2
+      # Enable this once CI reaches QEMU 8.2.3 or QEMU 9.0.0 
+      # - run: cargo xtask ci rs --arch ${{ matrix.arch }} --profile ${{ matrix.profile }} --package httpd --features ci,hermit/dhcpv4,hermit/virtio-net qemu ${{ matrix.flags }} --devices virtio-net-pci
+      #   if: matrix.arch != 'riscv64'
+      #   env:
+      #     # The buffer is sized to be smaller than the packets received during DHCPv4 address acquisition and thus exercise the buffer merging code path.
+      #     HERMIT_MRG_RXBUF_SIZE: 200
       - run: cargo xtask ci rs --arch ${{ matrix.arch }} --profile ${{ matrix.profile }} --package httpd --features ci,hermit/virtio-net qemu --sudo --devices virtio-net-pci --tap
         if: matrix.arch != 'riscv64'
       - run: cargo xtask ci rs --arch ${{ matrix.arch }} --profile ${{ matrix.profile }} --package httpd --features ci,hermit/dhcpv4,hermit/virtio-net qemu ${{ matrix.flags }} --devices virtio-net-pci --no-default-virtio-features